Alright, I have the cold piston slap which is gradually becoming louder with miles accumulated, no surprize there.

But what I've noticed is that the slap doesn't occur when the engine is started, but rather it develops as heat is built into the cold engine by driving gently, and reaches peak intensity about the time the temp gauge reaches it's "normal" reading. From that point, it slowly diminishes as the engine warms further, and is gone after another 2-5 miles of driving (depending on ambient temp).
Geek type question:
Why the buildup of the slap noise during warmup? I'd think the piston, being the primary recipient of combustion heat during the warmup cycle, would expand faster than the bore during that period and therefore be subject to a snug, bore-fit.
This puzzle rattles in my head at least twice a day during the winter.
