I have a 97 Yukon with the 5.7 and its been starting to get cold out and upon first start up when i put it into drive with my foot still on the brake I can hear a moderate knock from the lower half of the engine. It only does it if its colder that 40 out and it goes away mostly when it gets warmed up. Is this just piston slap that chevys are known for or could this be a rod knock. Also I did a valve job about a year ago and when i pulled the heads I was shocked to see the factory crosshatching on the cylinder bores. So it has been very well maintained and it appears to have had virtually zero wear over its life.