There is a hum coming from the fan in my HVAC unit - it is not the compressor, as I hear the hum when that compressor is on and off. I have 25 feet of flex duct with multiple turns between the HVAC supply and where the flex duct enters a spiral duct that runs the length of my condo. I can hear the hum coming out of the first few ducts especially well - by the time it gets to the later ducts, it's all but dissipated. I don't know how in the world I can still hear that hum 25 feet down the line and after multiple turns, but I can hear it plain as day. I *know* it's coming from the unit itself because when I get up in the closet where the air handler is located, I can hear it coming from the fan.

Does anybody have any suggestions on how to handle this? Almost as soon as the spiral duct comes out of the wall, there's a 90 degree turn so that the duct then runs parallel to that wall. Does anyone think putting a turning vane or two in that elbow would help deflect this sound? How about insulated flex duct?

Thanks so much!