Gear Howling

[PaHumBug]

A different thread about gearing was mentioning gear howling and I didn't want to hijack the thread so I started this one. I bought my 2005 used with 6" fabtech and 37's already installed, and it's done this since I've gotten it. It may be my imagination but I feel like it's getting worse lately.... At speeds between 20-40mph; If I am completely off the throttle or getting on the throttle, it sounds normal, but if I give it just enough throttle to maintain speed, it howls. When coming to a stop it howls once I get under 5-10 mph as well. At speeds over 40mph it doesn't seem to howl at all, or at least I can't hear it. I can't really tell where the howl is coming from (front, middle, back). What does that howl indicate? If it's not hurting anything it doesn't really bother me but I'm sure if there is noise it means that something is wearing out faster than it should.