At 93,000 miles my engine began knocking at startup just slightly. It is now pretty loud at 137,000 and lasts for about 4 miles until it warms up. I've read a ton about the piston slap. This is a pretty loud "thud" that seems to be just coming from 1 cylinder. I've heard GM's with piston slap and it is a lighter sound and it seems like more than 1 cylinder slapping.
My question is this: I'm taking off both exhaust manifolds in a few weeks to replace the studs, gaskets and manifolds. While the manifolds are off I'll look into the cylinders as best as I can. I know I wont see the rods but will piston slap damage show up that high on the cylinder walls or will it be below the exhaust port? How hard is it to drop the oil pan and check the lower end? Thanks.

 

You can't see anything that way.
Piston slap is a piston 'skirt' with to much clearence to the cylinder wall until heat expands the shirt and takes up some of the clearence, then the noise reduces or goes away onlty to return again at next cold start.
It's a wear thing.

 

Would a rod knock go away after warmup? Thanks.

 

Not normally. The same expansions don't take place.
Once the clearence gets excessive, there is no way to improve on it but possibly heavy oils and additives.

 

Thats kinda what I thought. I'm putting new manifolds and gaskets on due to bad leaks. I'll see if it helps at all. Maybe part of the noise is the leak although it seems that I can hear the knock and the leak separately. Thanks.