The extremely loud reverberation in the toilet bowl after you release a massive fart while taking a shit. This usually happens when you're trying to take a quiet, stealthy dump. Comes in 2 different forms:
1: Dryfire - Easily the loudest and most boisterous of the 2. Pure farts with no fecal matter; these happen from pushing hard or being constipated. These are so loud they could be heard from 30ft away or more with crystal clear clarity.
2: Explosive - These aren't as loud, but way nastier sounding. These happen with the occasional normal shit, and are 100% bound to happen with bad diarrhea. One of the nastiest sounds on Earth, but hilarious if you manage to hear them coming form someone else.
I was walking by the men's room at Taco Bell the other day, but had to stop and listen because someone had some intense bowl reverb. I came back to my table laughing my ass off.