just because you can here it with the stereo on probably means that you need a new stereo

- I have still have the stock CD player and it pretty much suks
anyways - it could be condensation dripping onto hot exhaust pipes or it may be the compressor clutch and could be that the compressor is chewing itself up - could be as simple as a clutch bearing as well -
if this is your problem there are two schools of thought on this. The first is to replace the compressor and clutch as soon as possible to minimize the amount of trash that ends up in the accumulator and other parts of the A/C system. The other school says let it go until you no longer get any cold air and then do the replacement. Why the difference? I had a GM compressor do exactly what your is doing for over year before it finally grenaded.
I wouldn't worry too much though, its probably nothing....