Dude, BE AFRAID!! /ubbthreads/images/graemlins/tongue.gif OK, maybe not afraid, but still... /ubbthreads/images/graemlins/crazy.gif
Open up the hood with the engine idling, then have someone inside hit the ECON button, if the sound disappears, and reappears with the ECON off, then take your car in to a mechanic. Trust me, it could just be that the coolant is low, which means that the compressor isn't being lubricated well enough. In that case, a purge and refill will square you away. BUT, and I'm speaking from experience here, if the compressor is the problem, it's better to get it fixed BEFORE it siezes up /ubbthreads/images/graemlins/frown.gif My compressor siezed while I was doing about 160kph on the autobahn, and by the time I got to the side of the road, the main belt and pretensioner were completely gone, and I was prolly minutes away from a fire under the hood /ubbthreads/images/graemlins/burningmad.gif the belt was smoking REAL bad /ubbthreads/images/graemlins/shocked2.gif and it wound up costing me about 800 before VAT /ubbthreads/images/graemlins/frown.gif to get it all fixed. (NOT to mention I had just gotten the belt and pretensioner replaced during a service about 3 weeks before /ubbthreads/images/graemlins/mad.gif)
If this IS the problem, sorry to be the bearer of bad news /ubbthreads/images/graemlins/crazy.gif
Hope it helps /ubbthreads/images/graemlins/beerchug.gif