moaning noise when climate control is on

[domchang]

my 87 560SL makes this low moaning noise, over and over, when the climate control system is on....it's almost too low to hear over the radio but not quite....heat and ac seem to work fine however. It's just an annoyance but am wondering if it's a harbinger of an upcoming failure in the blower perhaps?

anyone else's car make this noise?

update: turns out it may not be the climate control system, the noise occurs whether or not it's on. I am thinking its some kind of harmonic that occurs between 40-50 mph, does not seem to occur at higher speeds but then again road noise may be a factor also.