Not trying to be silly or whatever, but is your grey tank (nearly) full?
The reason I ask is that I recently was flushing my black tank and forgot to set a timer. As the tank filled, the stack running through the roof started making a clicking/popping sound as it was slowly being pushed up by the top of the full black tank.
(Since everyone will want to know - I managed to run out and open the drain valve before filling the trailer, roof or bathroom with smelly water.)