Any idea on what this knocking is 72 C20 350

[Stocker]

Have you tried to pin it down with a mechanic's stethoscope? Works better than a hose imho.

It doesn't sound to me like a cracked flexplate but I guess that's a possibility.... or loose flexplate bolts. As other have said, it sounds more like valvetrain to me.

[Chilly178]

No I do not have one of those at my disposal. What should I be looking for when I take the valve cover off? Is a little bit of play in a push rod or a rocker arm acceptable?

[AussieinNC]

Remove the passenger side rocker cover.

You need to manually turn the engine so that each passenger side cylinder comes up on compression stroke...you can watch the inlet rocker close off, then turn the engine 1/4 turn...

On that cylinder, check the rocker arm clearance to the valve stem...it should be just touching the stem.

Try and push down on the pushrod end of the rocker arm,,,if it goes down at all, you have a bad lifter.

If the rocker arm has any clearance over 1/16" the adjustment nut on that rocker arm has either backed off...or, worst case, the stud has pulled slightly out of the head. Either way...its not a simple fix...

What is the oil pressure with the engine running?

I have also heard a noise like this when the torque convertor bolts come loose...you can check by removing the flywheel cover plate if it fitted and check each of the three bolts and nuts for tightness...

While you are under there, grab the flew plate and try to move it rotationally back and forth...listen for a clunking sound...

If it clunks it could be a cracked flex plate...

[Chilly178]

Thanks for all the input! I ran the engine a bit more to give you guys a better idea. https://www.youtube.com/watch?v=4ReGCS4Jkg0

And then I also ran it again with the passenger side valve cover off.
https://www.youtube.com/watch?v=ft2xSmAX-_A

https://i.imgur.com/xuUcwM7.jpg

I'll check the flywheel sometime tomorrow.

[AussieinNC]

The rear rocker arm is not moving much at all...it is most likely a bad lifter or cam lobe gone...check the video where you have the rocker cover off...the rear rocker arm is not moving much at all...

[Chilly178]

I see what you mean.

[towbar]

Originally Posted by Stocker

Have you tried to pin it down with a mechanic's stethoscope? Works better than a hose imho.

It doesn't sound to me like a cracked flexplate but I guess that's a possibility.... or loose flexplate bolts. As other have said, it sounds more like valvetrain to me.

Sounds deeper and not as high pitched as rockers or pushrods to me. I'm in the line of thinking of the loose or broken flex plate. Also look for the flexplate hitting the torque converter cover (inspection cover). Quick 4 bolt check to rule these out. Good luck

[Stocker]

Originally Posted by towbar

Sounds deeper and not as high pitched as rockers or pushrods to me. I'm in the line of thinking of the loose or broken flex plate. Also look for the flexplate hitting the torque converter cover (inspection cover). Quick 4 bolt check to rule these out. Good luck

Yup, an easy thing to check. I find it a bit hard to diagnose sounds from a recording. When my flex plate cracked, it made a very deep hard knock and I honestly thought it was a rod bearing about to let go. The OP's video just doesn't have that same sound to me, but I would not bet any money on it either way. It's just too hard for me to tell.

[Chilly178]

I see. Tomorrow I will check the fly wheel and the bolts and as well as take off the two rockers to compare them for wear. Thanks again everyone, will post back tomorrow.