Why does my audi tt sometimes make a moaning metallic whine noise?

I think this may be a compressor/ac issue--it comes from under the hood. Sounds like metal on metal.Why does my audi tt sometimes make a moaning metallic whine noise?I think I know what your talking about, my A4 does something like that. Its just the pumps going, its nothing to worry about unless it sound like something is rubbing.