After reving, when the RPM's are going down, it sounds like my exhaust is popping or something. Also it sounds like somethings vibrating in the muffler when the car is at a stop.

i have a megan racing drift spec exhaust. Does anyone know what the problem might be?

 

The popping comes from raw gas in the exhaust igniting.This happens as well when you rev it up while driving and let the throttle off quickly.
The vibrating noise you hear could be a heat sheild touching your exhaust ,but if your sure its coming from the muffler than the baffle is most likely loose.