Jeff:

Consider blocking off your hot air source to the mixer box to see if your "twack" goes away.   If it does, at least you will be pretty sure that the problem is heat related and is inside the mixer box.

BTW, where are the sonic valves for your hot air source located?  I understand that some engines had the sonic valves located on the hot side of the intercooler which produced really hot, hot-air.  If so, super hot air might  be the culpret.

Just a thought.  Bill Hogarty