Probably just carbon buildup or some corrosion from age. I can hear it rattling inside when I shake the exhaust. Maybe it's an excuse to buy something new. I've read of tricks like spraying engine degreaser in there to dissolve carbon buildup into little pieces that will eventually blow out the exhaust.I don't think you can remove just the mufflers (I don't have any and can't remember the stock setup). I think you're other option is to drop the exhaust, but I don't think that will solve your issue. Are you sure it's not the muffler attachments that are loose/rusty? Trying to think how you might have loose particles in your mufflers loud enough that you can hear them. My hunch is that some other part of your exhaust is loose.