From what I've read around the net, they sound like a cheap copy of a Flowmaster 40, which sucks at best.
I would pony up the $$ for a Magnaflow before I would consider a Flowmaster muffler, but that's just my opinion. Some guys like the Flows. Just remember, you get what you pay for
These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.