Popping sound in exhaust

3310 with 2 metering blocks is a 4150 model. Usually these 3310's are rated at 780CFM for some reason. OEM carb for a BB Chevy one year. Metering plate is a 4160. We had a guard mount 4sp IH 2 ton truck that would backfire every time you let off the gas in gear to create a high vacuum and then blip it. Was like a cannon after the mufflers blew out their packing. The driver would pop that thing about 10 seconds from your post to either wake you up or get you up off your ***! Motor T guy said we probably had a leaking valve