1996 Sunfire muffler annoyance

I have a 1996 Sunfire GT standard with 123,500km on it. I've owned this car now for 11.5yrs and maintain it regularly. Since the summer I've been hearing a rumbling from my muffler (one of the two). I've had it checked out twice now (muffler shop and GM dealer) and both times the auto shops have come back with the answer that there are no holes so it doesn't need replacing. I know I could wait until it rusts through, but the noise is driving me nuts. I don't intend to sell, but keep it for years. What would be causing the noise internally in the muffler? How much would it cost approx. to replace it with quality parts with dealer labour $s (not cheap junk parts that will last a year - had that problem already with the water pump)? If I wait, how long before it does rust through (I'm assuming that's the prob)?