It sounds more like you dropped a screw behind the back of the MOBO and the pop was the transfer of some electricity into the case.
I know its a pain, but wiggle the case (gently as to not dislodge anything) if there is anything loose you will hear it wiggling about, you could take the MOBO out and check, but this is a pain if you have alot connected to it.
The only other thing is if one of the PSU wires is frayed and shorting on something, either way its very dangerous and needs looking at :eek:
Wear rubber gloves, and huge wader wellies :D (If you choose to do this you must upload a picture)