I was in a DM, I had a warrior and was firing the Ion Gun while moving. I ran across a laser powerup while firing the ion gun. The ion gun fire sound contunued to play even when I stopped firing, even when I switched weapons and fired different weapons.
It seemed to forget to stop one weapon sound when a different one loaded into its slot.
FWIF
Turrets will also do this if they are firing miniguns when you drop a chain upgrade on them. The hard fix is code, the easy ingame fix is drop a mini (or the original weapon) on the offending unit and let it fire it, then upgrade it while it is not firing.
Sorry there is nothing better yet, it is most likely fixed in current 1.3 private beta builds.
It has been fixed in the private beta. I know its an annoying bug. I haven't tested the morphing bug in a while, does it still occur. The weapon would loop in a similar way if you were firing ion guns then you morphed at the right time.