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?