I have the same problem except it only occurs when the car is cold. I hear it mostly at low RPM's. A kind of spooky howling sound. Maybe the car has the halloween spirit. Im sure its a bearing on the front of the engine which includes; water pump, AC compressor & idler, Power Steering pump, Alternator or air pump. I have not been able to figure out which one it is. Might just have to wait till it fails. I was going to get a stethescope and see if I could find it that way. Im just praying its not the water pump (again).

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k