VDiddy

I just built and installed a fresh 408 in my CTS-V and there is a strange noise that appears to be coming from the front part of the oilpan, about 6 inches behind the crank pulley. It doesn't sound like knocking or scraping, very difficult to describe. It is a rythmic noise that seems to move with RPM but is not always present. Oil pressure and temperature are fine. My machinist listened to it with the car on the lift using a stethoscope and said he thought is was potentially a bad lifter making a noise and the harmonics centered in the oilpan.

Has anyone else experienced this? What was the cause?