My mechanic took my car for a spin after putting in the new built shortblock. At the time he also installed my new GSC S3 (<-- could be relevant). He said around 3k rpm and under there is a very slight "knock." There is oil pressure...any ideas?

 

Forged Piston knock when cold, or knock all the time warm and cold? Lifter noise or lower end?

 

do you have the motor specs? clearances?

 

If it stops "knocking" after the engine warms up the knock is likely just the pistons before they expand. That would be normal for an engine built for high performance.

 

sounds like its your lifter.. bleed them.... should fix your problem

 

just picked the car up...it seems to be my twin disc (comp twin) making the noise. it seems to only happen when i accelerate from 2000 to 2800 rpm, other than that no noise also i have put almost 100 miles on it, so if it were the rod bearings, it would have already happened. i think my mechanic was just being overly cautious.

 

really?

it amazes me how people comment on things on here and have no idea what their talking about...

op sounds like the clutch..

 

Noise also goes away when clutch is depressed in same rpm range where it makes the noise.

 

In reality though, trying to diagnose an issue with a car over the Internet is itself a futile effort. However you can get good ideas on things to check.