I decided to exhaust my sump pump in to the municipal drain. My first attempt resulted in too much noise due to no vent stack readily available. I next installed a cheater vent (short stack pipe with a rubber flap) which also generates too much noise (sound like a gastrointestinal disorder). I tried to sound-proof the cheater vent but it still sounds like a long muffled fart followed by a clunk. Any tips would be appreciated.

Thanks... Dave

Not what you asked but are you 100% sure hooking your sump up to the city sewer is legal in your town, it isn't in most towns. Your cheater, is it an AAV like a Studor?

I live in an older neighborhood where most everyone is still connected to mains. The cheater I installed is a cheap one, and your response has prompted me to consider replacing it with something more robust. I seem to recollect the reason I went with the cheaper version is the lower height.