E36 M3 Evo... Odd sucking noise?

[thegreengoblet]

Normally at it's loudest from cold and only when applying reasonable throttle movements and only when moving - speed irrelevant. Sort of sucking noise but has a sort of metalic tone to it. Sort of pad to disc scrapping sort of sound. You could believe it's air in a liquid passing through a point somewhere that's causing the noise. Seems to come from behind the dash directly in front of the driver, although it could be a little further and in the engine bay. Doesn't do it at standstill with revving?? Really strange. Seems to got a lot quieter when warm, but I can still hear it. Any ideas?

[joesnow]

Something to do with aircon?