when your mufflers are going bad??
because i wonder about my flowmaster, the sound seems like it changes big from day to day.. somedays its loud other really quiet. some days it rumbles or they don't.. should i be worried??

 

that could be more indicative of a problem with one or more catalytic converters. the sound difference is probably just your perception more than any actual condition but if a cat has crumbled then you're likely to get all kinds of sound artifacts. mufflers are easy to tell if they're bad... they'll have holes in them from corrosion or you'll hear a piece banging around inside them (especially with flows... they're good forever really).