Flowmaster question
Yes I have, all joints are tight and no holes i could see
No holes you could see...Flowmasters are chambered mufflers, if the chamber fins are degrading internally, it can cause the pop you are hearing. My old muffler started doing this before I replaced it. After a hole rusted into it, I could see the internal fins were degrading and had some thing spots, and probably an internal hole I could not see. You're probably getting the same thing.
How long has it been on the truck? Months? Years? The source of the pop could be any number of things, my FM muffler was about 6 years old when it started to go.
It could just be the setup. I know it's common on other cars (C5 Corvettes for example) with catback setups - some have the pop and some don't. You could replace the muffler and see if that fixes it. Try a magnaflow. Shouldn't cost you more than $100.



