So I had a new-used motor put in my 04 LS V8, this new motor had only 39,000 miles on it when it was put in. Ever since it was put in it has had this motor noise, where it either sounds like an exhaust leak or a small ticking in the motor when it is cold, or when I start it up even after it has been driven for some time. The only time it doesn't make the noise is after it's been running for about 10 - 15 minutes, and it is at operating temperature. If I shut the motor off, and start it back up it will make the noise again until I start driving it. The noise is most notable when their is a load on the motor.