I have a vibration tapping sound coming from under the hood for about the first 2 minutes of cold start-up. Do you guys think I have some loose rockers or an exhaust rattle? It only happens on start-up when the car is cold. If its already warm on a restart then it doesn't make the noise.

 

Piston slap is common and people often mistake it for lifters/valvetrain or exhaust type vibration.

Ive seen many motors start piston slap out of nowhere and just suddently develop the noise when the motor is cold. Sometimes it doesnt always fully go away even when the motor warms up and the motor runs fine for 10's of thousands of miles with no problems.

 

might wanna check your oil pump. do you have pressure? it could be that when the motor is cild it's having trouble getting oil to the top of the motor.

 

I have 60 lbs. of oil pressure at start-up and I know its not piston slap. This is a lot louder than piston slap but it goes away quite fast.

 

I've seen many LS1's that people "knew it wasn't piston slap" because it was alot louder than typical piston slap. And after they had torn down valve train, replaced lifters etc etc finally realized that it was piston slap

 

Piston slap is characterized by a sound similar to knocking two pieces of hollow wood together.