Not to sound like I am second-judging your response, but I may have been unclear in describing the problem. It occurs irregardless of temp and my aux fan seems to operate normally. This noise is different than that of the aux fan. It could better be described as you're driving along and rev in to higher RPM's reaching around 4000 then as though someone was to massively restrict pressure in the exhaust or something. It almost has a hummm as well as air-noise... Or it sounds like an additional belt is kicking on, but it is not. Sadly my a/c is not working at the moment, nor has it for the past year...but this noise is new. Any ideas? sorry if i'm unclear, just difficult to describe. Thanx a lot...chris..