The dreaded knocking noise?

[Jonlopes]

My engine is making a noise I hate to even think the “K” word but it is more of a knock than a tick. Kind of like if you were knocking your knuckles against a door or something.

Here is what I know so far:

1. It is the stock 396/ 402 that was rebuilt in the 1990’s. The rebuild supposedly has 30-40K miles on it.

2. It is not that loud, I cannot hear it while I am inside the cab and driving unless I am in the garage (sound bouncing off the walls?). you can definitely hear it when the hood is open.

3. It happens while the engine is idling and speeds up as I rev the engine. It happens in park, reverse and drive.

4. I drained the oil and could not find any metal shavings. I added a quart of Rislone when I refilled it with fresh oil hoping it would quiet down a noisy lifter but I have only been able to drive it about 10 miles since then. No change yet. Should I try motor flush?

5. The truck has stock iron manifolds and I cannot feel any leaks but the passenger side manifold flange is attached to the pipe with only 2 of the 3 bolts. If it was leaking from the flange would it be a knocking noise or a rumble? I purchased some new studs and will try and get this taken care of this weekend.

6. I read somewhere the rockers can be hitting the valve covers. The stock valve covers are gone and there is a set of cheap chrome covers on it right now. They both have a slight leak so I will be replacing them with a nice aluminum set I have. There are no dents in the thin Chinese sheet metal.

7. It runs and drives smooth, plenty of power. It idles pretty smooth too. If I did not know the noise was there I would be very happy with the engine.

8. It is really difficult to count but it seems to be about 5-6 times per second at idle. It is definitely not 10-12 times/ second. does this mean it is going with the cam not the crank because the crank turns twice for each turn of the cam?

9. My aftermarket oil pressure gauge shows about 30 psi at idle and 60-70 while driving.

Are there any good things to try or should I take it to a mechanic who better understands internal engine issues?

[kingsolver72]

Wrist pin?

[Jonlopes]

Originally Posted by kingsolver72

Wrist pin?

Can that be checked without a full tear down?

[kingsolver72]

Originally Posted by Jonlopes

Can that be checked without a full tear down?

With the engine running pull the spark plug wires off one at a time. Listen to see if the knock goes away on any of the cylinders.

[michael bustamante]

tighten and put in those exhaust manifold bolts. it does make a ticking noise but not a engine kind of tick.

[Jonlopes]

Originally Posted by michael bustamante

tighten and put in those exhaust manifold bolts. it does make a ticking noise but not a engine kind of tick.

Manifold to head are all tight it is just the passenger side flange where the manifold mates to the exhaust pipe that is missing a stud. It looks like the threads are still there so I will be adding that stud this weekend whether it fixes this problem or not.

[mikecaddy500]

fuel pump?

[Jonlopes]

Originally Posted by mikecaddy500

fuel pump?

hmmm, yes that would be once every two revolutions since it is running off the Cam. They are pretty cheap. I wonder if I should just replace it?

[piecesparts]

Originally Posted by mikecaddy500

fuel pump?

Agree with the fuel pump. Try replacing the pump and then see if the knocking still exists.

[CST10]

I had an issue kinda of the same. Changed the fuel pump and it wemy away. The old fuel pump return spring on the lever arm was broken into two pieces.

[Jonlopes]

Originally Posted by CST10

I had an issue kinda of the same. Changed the fuel pump and it wemy away. The old fuel pump return spring on the lever arm was broken into two pieces.

Fuel pumps are cheap enough that I will not feel bad if it is not the issue. Also I really need to get rid of the rubber hose going from the pump to the carb. This will be my excuse to change it out.

[mattfranklin]

If you put in heavier oil does the sound decrease? My 350 got a little better when I switched from 10W30 to DELO 15W40. But the the noise came back when it warmed up. I'll be tearing into mine later this summer. My guess is rod bearings. I'm hoping to get by with just a new crank kit.
Posted via Mobile Device

[jessemthompson]

Trying posting a short video. Easier to diagnose if we can hear it.

[Jonlopes]

Originally Posted by jessemthompson

Trying posting a short video. Easier to diagnose if we can hear it.

Good idea, I will do so this weekend.

[71 super]

I have a 402 that has ticked from day one of rebuild. It has been pulled apart so many times it should have zippers. The very last thing discovered after throwing good after bad was a loose valve guide. Is it on driver's side near front? My with a scope seemed all over the place and could never nail it down. Went away with acceleration but returned at idle no matter the oil viscosity or additive base added.

[Boog]

This knock or hard peck can be caused by a number of things. Failing lifter, fuel pump ( always buy AC Delco replacement ) as some of the other brands have weak springs. Lesson learned. Carbon buildup on piston top and combustion chamber can cause a knock that sounds just like a rod knock. Been there too. Loose torque converter bolts make a similar knocking sound. That one made a real hard knock.
If you can use a long length of hose or a long screwdriver or dowel rod to narrow down the location of the sound it will help to diagnose it. Hold one end to your ear and move the other end around the engine while it is running.
This will help to tell if the sound is down low in the engine or upper end, left or right side.

[beautimus]

[QUOTE=Boog;7194765] Carbon buildup on piston top and combustion chamber can cause a knock that sounds just like a rod knock. Been there too. Loose torque converter bolts make a similar knocking sound. QUOTE]
I am gonna get flamed for suggesting this but, here goes...
carbon build up is easy to diagnose and eliminate, pull a spark plug first, to see if its got a lot of carbon on it. if the plugs are gunked up, then the cylinders are likely to be also. use a soda bottle to SLOWLY drip water into the carb of the running engine. go very easy and don't let the engine go dead. just a drop or two at a time., over the course of 5 minutes or so, to use up a half of a soda bottle of water. then go out and drive the thing for a few miles. I have used this trick to clean carbon from cylinders and to remove carbon from the spark plugs on several engines over the years. oh, you will want to remove the breather and work the throttle by hand as you do this, to help keep it from going dead, until you get the hang of it. if the engine stumbles, your going to fast. that and checking the torque converter bolts are zero cost things to explore. Always go for the simple and cheap stuff first right?
best of luck and keep us posted on what you find.
Tony

[old Chevy guy]

First thing that came to my mind was fuel pump. 350 in my 69 Camaro had the death noise and was only that. My dads truck had an awful knock that turned out to be loose torque converter bolts but if I remember correctly it only did that in neutral or park. I guess in gear there was pressure on it so it didn't rattle and knock.

[Jonlopes]

I ordered an AC Delco fuel pump and also a new rod for the pump. I will install them this weekend, this will also give me a chance to add a steel line from the pump to the carb.

I will then check the torque converter bolts. The PO told me that he installed a new flex plate because the old one had bad teeth.

[Boog]

Once those converter bolt holes get wallowed out its time to replace the flexplate too.

[jdw]

You've also got the brake issue in another thread. Are these related?

[Jonlopes]

Originally Posted by jdw

You've also got the brake issue in another thread. Are these related?

I am not sure. The knocking noise is coming from a moving part and it happens while the truck is in park.

Though I wonder if I have bad lifter(s) and/ or a cam with worn down lobes will the truck have lower than normal vacuum? That in turn would give me the funky brakes I mention in the other thread.

That would be cool to solve both issues with 1 fix!! Does anyone know if lifter/ cam issues would show low vacuum?

[jdw]

Or your torque converter is busted and isn't holding the brakes?

[Jonlopes]

ok Thank you to everyone who gave advice! I have lots of things to try so here is the plan for this Sunday (my one free day to work on my hobbies)

1. check the manifold vacuum for either low vacuum or fluctuating readings (What is a normal reading?
2. change Pass side exhaust flange doughnut gasket and add the third stud
3. Check Torque Converter Bolts
-Check starter while I am down there to see if it is not adjusted correctly and hitting the flex plate.
4. Change the fuel pump with the AC Delco replacement I just received in the mail
-install steel fuel line from pump to carb
5. remove leaking cheap chrome valve covers check the rocker arms + springs for any obvious problems and reinstall aluminum covers with rubber gaskets
6. change the oil 10-40 to 20-50 to see if the thicker oil can help a weak lifter.

Hopefully I am not too ambitious for how much I can do in an afternoon but I will report back with any successes or failures.

[67 chevelle]

faulty pcv valve will knock [i have experienced this], brake booster could be similar ,[thought i have never experienced this] , some other things i have experienced that made a knock were , a broken piston ring ,rockers hitting valve covers , torque converter bolts hitting transmission ,, and of coarse the obvious crank bearing , with a bad crank bearing you will loose oil pressure