I really appreciate all of the ideas y'all are giving me. I'm definitely going to mention them to the mechanic.

They did replace plugs when they installed the motor. The knocking/popping is worse around 2700-3300 rpm, and then it gets a little softer, but it does not go away.

My guess would be a defective lifter or a bad valve spring....

money trying to get out of your wallet

Boy, Luke, if you only knew how true that is!

Thanks for all the input.

A failed exhaust lobe or lobes will cause exhaust to stay trapped in the cylinder during the exhaust stroke. Then the intake valve opens and the hot exhaust comes out through the intake.

A 454 we once had a slight, somewhat random, knock when cold that cleared up when the motor warmed up. Bad exhaust lobe.

An exhaust rocker that loosened up could cause similiar symptoms- if found, don't just tighten it up!!! a bad lobe will cause clearance to open up also, so check lift afterwards.

I have also had knocking sounds I couldnt quit seem to locate turned out to be the drive .... If you do alot of ski or tube stuff and have hammered out of the hole alot your forwad gear and/or thrust bearing migt be ready to bust. If your shure sound is from the motor one thing not mentioned is it could be detonation/ timing issue.

We'll definitely tell the mechanic to check the timing. Talked to him today, and they still can't figure out problem. This is getting frustrating.

check the firing order and double check the plug wires to make sure they are going to the correct plug.