I have a 2001 nissan sentra...there's a rattling sound under my car towards the middle front....around i guess the muffler thing - long silver thing that goes to back of car...it's getting louder and louder...do you think maybe a bracket is lose or something? my daughter is embarresed when i pick her up from school...i'm afraid this is going to be a costly ordeal...and i don't want "sticker shock"...

SOURCE: My car makes a rattling sound under the car

Hey Thealmeidas!
Yep, there's a good chance some part of the exhaust system has come loose and is rattling at certain RPM's. Start fault-tracing by checking if any part of the exhaust seems loose to come in contact with anything, or if there's any loose clamps or similar parts hanging onto it. Another quite common reason for rattle is that some part of the exhaust manifold, such as a bolt or hose/pipe has loosened (often due to rust and/or corrosion).
Another possibility is that it's somehow related to the transmission (since you mention putting it into gear). If so that would be another subject.
Good luck!

SOURCE: Exhaust muffler got louder. No holes in muffler.

Probably corrosion inside destroying the baffles and letting the wadding to break up. Sorry new is the answer.

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SOURCE: rattling sound from the exhaust

The metal shielding around your catalytic converter could be rattling or the rubber donut that holds the rear tailpipe could be bad. When your car is cold shake your tailpipe and see if you can reproduce the noise. If you can your tailpipe donut is likely bad. Three dollars at any auto parts store. If it does rattle when you shake your tailpipe then the heat shield for your convert could be loose. You need to tighten it up another piece of metal between it and the converter to keep it from rattling. If you hear the rattle when you go over bumps too then your exhuast is probably, loose. Crawl under there and stake shaking things, I am sure you'll find the problem

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You could have a loose bolt on your exhaust manifold (this happens sometimes with Japanese cars with little reason behind it). If you can, try tightening all of the bolts on the manifold and see if that helps. If you are not very mechanically inclined and tightening the bolts does not totally resolve the issue, I recommend taking it to a mechanic -- just mention that you think it is the exhaust manifold, or the exhaust manifold seal. If the seal is worn out, you should have it replaced and that should resolve your issue.
The reason it only happens at idle is that the engine has a specific vibration to it at low RPM's ... this is a different vibration than when driving ... stepping on the gas also increases the pressure in the exhaust system, minimizing the vibrations. You should be able to get it fixed for a pretty low price, but I would recommend doing it soon so you do not allow collateral damage to the rest of your exhaust system.