Suppose I'm loading for the 6.5x55 and I want a 140 at 2500 fps. Will I get lower noise report with a heavier charge of a slow burning powder or a lighter charge of a fast burning powder? Assume standard primers.

Anyone have any input or, better yet, some lab results? TIA.

Slower powders have higher pressure at the muzzle. More pressure, more report. HTH, Dutch.

My experience has been faster powders give less report, up to a point. I don't have a 6.5 right now, but from what I can remember you are a little past that point here in my experience. If you were looking at 100 gr around 2000 fps is where you'd hear a very noticeable difference IMHO.