If you select the Piston Power talent and then switch away to a different talent, the Piston Power buff remains. Since the buff can’t be triggered and won’t go away, every heavy melee / Grudge-raker melee attack has the Piston Power sound effect. Switching back to Piston Power and activating it, then quickly switching off doesn’t help because the buff stays and recharges, regardless of what talent is selected.
This bug appears to be cleared by entering a map, so it’s seemingly only an issue in the Keep.
However… it also seems Piston Power can only be triggered by actually connecting with a heavy or Grudge-raker hit, meaning every non-hitting use of those attacks has the sound effect until triggered and on cooldown, which feels excessive.
A similar issue exists with Leading Shots where the shot counter remains on your buff bar after the talent is unequipped.