Strange Blower Motor Hum

[dadysev]

Hello, before everyone jumps and says its "got" to be the blower motor, here is what I hear on my 2007 4.8
if i'm just sitting in my truck and the fan is on the lowest speed and I hear a hum which goes on and off and its very faint but can be annoying. So I say to my self ah crap the bushing or bearing in the blower motor is done, but if I increase the fan speed the frequency should change and possibly get louder.
Nope!! i can go almost 1/4 to almost 1/2 of the way and still hear the noise at the same frequency and its not any louder.
Turn off the fan, which obviously shuts down the climate system and it goes away.
I'm guessing there maybe a flap motor or something acting up. i have tried every button, flap position, etc. and still nothing.
Has anyone had this happen to them ?
Thanks.