The differences are stock cast pistons have less bore clearance and they get up to temperature quicker because they have less material. Forged pistons are looser in the bore, the metal is denser and usually thicker, so they take longer to warm up. When they are at operating temperature, they are still looser than cast pistons and thus potentially noisier. The extra clearance is taken away under heavy load when the pistons get hot, but for most of their operating time, forged pistons are not as close to the bores as cast pistons are.

 

Your truck sounds like a damn diesel on a cold start

 

But it runs good!