A friend of mine went to the local salvage and picked up a 350 SB for a good price. It was very low mileage and was a pre 80 SB that was rebuilt they said. We installed it and it runs great except he has a popping sound on the one side of the exhaust. It's not a miss but pop- pop. I told him I think it's a valve. What do you guys think?Thanks for your help.