PCPs are a blast...

[keithy]

Loud pcps are usually due to wasted air following the pellet I suspect turning the reg down may help?

[angrybear]

It might, or it might slow the MV,
it may be that the reg pressure is spot on & it wants a little less hammer spring, or a little more valve return spring, so the valve is open for less time &/or snaps closed quicker.