I've found a few small (dime-sized or smaller) holes on the outside of my muffler. As far as I can tell, there's no exhaust coming out of those holes, nor is the muffler noisy. So is it time for muffler replacement? Is the muffler a multi-layered construction, meaning that holes in the outside layer are not necessarily a big deal, or are exterior holes a sure sign of impending CO leaks into the cabin? Opinions please!