Loud knock at idle!!

[Phorbes]

I have my 71 c10 with 454 swap in it. Today I was making a dump run and when I pulled into the bay to dump my trash I noticed a knock that just got louder and louder.

Metal on metal, quite loud and irregular. Like something is floating around bagging side to side.

I pulled out of the dump and drove her home and noticed that there’s no knock or any irregular sounds when she’s under load. Just while idling...

Any ideas what this could be???

Thanks guys
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[Wrenchbender Ret]

It might be something got into a cylinder. A hunk of carbon might have broke loose or an object came out of the carb or air filter. Also a fuel pump can make a noise like that.
George

[B. W.]

Well, let's hope it's not in the motor. The first things I'd check are the exhaust pipes, BB exhaust can be a tight fit, the motor moves around causing it to hit. On that note, check the motor/trans mounts. (The BB trucks have different engine stands on the crossmember to give more clearance, if you used SBC stands things are going to be tight)

Check the torque converter bolts. Wrong bolts (too long) will bottom out & be tight but the converter is still loose.

Also inspect the flex plate for cracks. I have had several flex pates crack over the years. they can cause odd, irregular sounds.

[Phorbes]

Ok this is helpful. I don't think it's inside the motor since it runs perfect when accelerating or just cruising. It's when it's idling that it sounds like plate is getting bounced up and down, side to side, with no measurable rhythm. Its very loud too... like a ball bearing bouncing around a metal cup. I certianly hope is a motor mount or something to that nature.

Originally Posted by B. W.

Well, let's hope it's not in the motor. The first things I'd check are the exhaust pipes, BB exhaust can be a tight fit, the motor moves around causing it to hit. On that note, check the motor/trans mounts. (The BB trucks have different engine stands on the crossmember to give more clearance, if you used SBC stands things are going to be tight)

Check the torque converter bolts. Wrong bolts (too long) will bottom out & be tight but the converter is still loose.

Also inspect the flex plate for cracks. I have had several flex pates crack over the years. they can cause odd, irregular sounds.

[kwmech]

My money is on a cracked flex plate (flywheel) or loose torque convertor bolts

[Phorbes]

that would make sense since they would be locked up under load right?

Originally Posted by kwmech

My money is on a cracked flex plate (flywheel) or loose torque convertor bolts

[Marine-58]

I had a 350 once that the flexplate bolts would loosen up over time. Just unbolted the torque converter and it would move back far enough to get to the flexplate bolts with a wrench. The knocking was only at idle and sounded very close to the noise a rod would make.

[MARKDTN]

Originally Posted by kwmech

My money is on a cracked flex plate (flywheel) or loose torque convertor bolts

me too.

[hugger6933]

My first thoughts were flexplate or converter bolts also

[BossHogg69]

I had a flexplate do nearly the same thing on a '99 Tahoe, 5.7L. It had a small crack in it and would rattle like crazy until the engine got warmed up, really noticeable at idle. Found the crack & replaced the flexplate, no more rattling.

[Phorbes]

I figured out the problem is but I don’t know how to fix it. There’s a floating crankshaft bearing that’s just rattling back and forth. See video link.

I don’t understand the purpose of this part or how to make it stay put.

Any insight ??

https://youtu.be/jfw1vDFuz5c

[NoNeck]

That is your harmonic balancer. It absorbs engine vibrations and you do need to replace it. There is supposed to be a rubber ring between the sections of metal. It is missing. You will have to remove the crankshaft pulley and belt and remove the large crankshaft bolt that holds the balancer (or dampener) and remove it using a wheel pulley.

I would replace it with a new, quality balancer. Summit Racing sells nice ones as well as NAPA. I would pick up a new keyway too to replace the old one. I hope this helps.

[Marine-58]

i would also replace the seal in the timing cover. Use a install tool to put the new balancer in as well. Most auto parts places like O'Rielley's have a tool loan program so you don't have to buy these.

[Phorbes]

Thanks guys. I think I’ll just hire someone to do the work since this is going to take some special tools and a good amount of time.

Thank you so much for the help