Alright, So I'm driving to work. I'm stopped at the red light to go on the freeway. It turns green so I accelerate. I'm accelerating more than I would on a regular street but I didn't floor it. Anyway, all of a sudden my exhaust sounded REALLY loud. It sounds like I have no muffler or something. I pull in to a gas station but my muffler is still there. It's loud at idle and gets louder the more I push down on the pedal. The sound sounds like it's coming from under the driver's feet. I looked under the car where I think the noise is coming from and I didn't really see anything. It looks like a bolt may be missing from about the same spot the noise is coming from but I don't know if it's related or just a coincidence. I don't remember hearing anything falling off my car when it happened. No warning lights or anything. Any ideas?
Probably a cracked/rusted out weld somewhere. Could be a hole that finally rusted through. Sometimes the bolted together joints rust through. Typically results in a cat-back exhaust replacement. Can end up being full system replacement. Rarely can be fixed in just one location.
I agree, it is likely a hole in your exhaust. I have a 99 A4 and get an exhaust leak about once a year by the flex pipe near the front. If a CEL didn't come on, I couldn't imagine something mechanically wrong that would cause it. Easy way to check is to jack the car up or put it on a lift while running and see if you feel air being blown somewhere along the exhaust. DON'T TOUCH THE EXHAUST, it will burn you. Just run your hand 6-12inches away.