I have had my durso in the tank for over 2 years. Never had any issues with it. Now, the past few weeks, it is making some strange noises. Not like the flushing or gurgling you would expect, but a sound like someone tapping a kettle drum with the reverbrations and everything. It is really LOUD at times and at other times not so much, but still there.
I shut my pumps off and removed the drainpipe all the way to the sump. It isn't blocked. I tried drilling another breathing hole in the cap but that didn't help either.