A vaginal secretion of air (queef) of unfathomable magnitude. It is of too low of a frequency for human ears to hear, however it has been felt on a handful of occasions. The only known members of the animal kingdom known to be able hear muff thunder are Humpback Whales. Further research is being done as we speak
No, I was having really intense sex with Cindy and she let out some ridiculous muff thunder and rumbled my lamp off of the nightstand and it fell right on Simba's head.