396 big block knock after warmup

[maroman]

could be a bad fuel pump.

On other cars I am more familiar with the fuel pump or vacuum pump ROD gets cupped from not turning properly and thumps. If you remove the rod and grind the end flat it fixes it. I don't have near as much experience with Chevys as I do with some others. DO Chevys ever do that?

[1stgen68]

Can't tell which side it's coming from. I'll need to start with a stethoscope and pull the surp belt like others suggested. 

[L78racer]

could be a bad fuel pump.

On other cars I am more familiar with the fuel pump or vacuum pump ROD gets cupped from not turning properly and thumps. If you remove the rod and grind the end flat it fixes it. I don't have near as much experience with Chevys as I do with some others. DO Chevys ever do that?

i have not seen that condition in a Chevy but a bad pump itself will sound like the noise in the video. could be other things too, of course.

[1stgen68]

Before removing the surp belt i got under the car on both sides and could definitely tell that the knock is coming from the back side of the motor somewhere. I can hear it just as equally on the driver side too.

[maroman]

Maybe we need some pictures?

[Kelley W King]

If you have an auto it might be the converter bolts. But I would do the belt and eliminate the others first.

[1stgen68]

Yes Sir it's an automatic.

[camaronut]

That noise is a strange one for sure.......to me its sounds like a rattle in the exhaust when it gets warm and the metal expands.  A friend of mine had a muffler that rattled when it got hot.

Another thought........could the teeth of the starter be real close to the flywheel?  Maybe the starter gear is not retracting like it should.

[crossboss]

If you have an auto it might be the converter bolts. But I would do the belt and eliminate the others first.

+1. Also, maybe the flex plate/flywheel bolts are loose.

[dutch]

That noise is a strange one for sure.......to me its sounds like a rattle in the exhaust when it gets warm and the metal expands.  A friend of mine had a muffler that rattled when it got hot.

Another thought........could the teeth of the starter be real close to the flywheel?  Maybe the starter gear is not retracting like it should.

Does your exhaust on the passenger side still retain its Heat Riser ?  They always clattered on any big block I ever had and I always tied them wide open with some wire to get rid of the noise if I was keeping the stock exhaust system on any cars I had.  The springs on them are supposed to open with the heat generated and often the spring either isn't strong sufficiently to hold the flap fully wide open once the housing gets warmed up and the flap inside the pipe just ends up fluttering around clattering with the various exhaust timing events and making quite a metallic clattering sound similar to what you have going on..

[dutch]

OOPs I see from the video you have headers - which makes the Heat Riser theory mute.. but check the flange bolts at least for tightness just to make sure it isn't one of them rattling around loose..
Or your emergency cable or adjuster hook not hitting against the exhaust somewhere also!

[olddragracer]

Loose flywheel to torque convertor bolts usually make an irregular knock at or just above idle especially when the engine idle is rough. Most of the serious engine problems have a regular beat to the knock. Loose or cracked pistons usually knock when engine is cold and idling. Wrist pin knock is most often heard with engine hot and idling. Connecting rod knock can be best heard when the engine is accelerated and the RPM goes over its peak. Main bearing knock can be heard when driving and under load, sounding like a heavy rumble. Valve train noise at the rocker arms is more of a tapping noise. Rocker arms hitting the valve covers is usually a lighter tapping noise. Yes there are many other sources for engine noises. These are some of the most common heard knocks when the fan belt is removed.

[JKZ27]

Check the flexplate for cracks.
What does it do off idle or when revved? Or, when in gear, does the sound change?

[1stgen68]

https://youtu.be/OyMoi1xAqak

This sounds like the same knock that I'm hearing under the car.

[Mike S]

Sounds like in the torque converter area if not the converter itself.
Is there a dust shield that is removable to see the flex plate?