Shorter or chopped exhaust systems have a tendancy to afterfire. If the system is full-length then check engine tune and exhaust system clamps/flanges.

I knew a guy that had a F-100 with a granny 4 speed and a 351M. He had glasspacks and a serious knack for messing with people. He would drive by the gas station I worked at in college late at night. He would get in front of the station, cut the ignition off then cut it back on. This would produce two shotgun blasts that would send the customers diving for the floor, doughnuts and coffee flying about.

The cops actually came flying up in the parking lot just after he left. Came in the store and talked to me:

"we had a report of gunshots fired"
"actually, it was a westbound **** ."

This happened about two more times. The forth time the cop comes in and says:

"green truck?"
"yup, F-100 in regress"

Preaching the Hot Rod Gospel according to the 4-stroke apostles:

Suck, Squish, Fire and Fumes