Everyone thanks alot for your input. I took AMGLovers advice and here is what happened.

1.) The noise of the hum wasn't affected by the rpm increase or decrease.

2.) the heat shield/wheel well molding etc. seemed perfectly and tightly attached.

3.) when I retracted the spoiler at speed after it automatically rose the noise stayed the same.

And perhaps this last one will shed light on the problem.

4.) When i shifted into neutral at speed while the humming noise was occurring it would immediately stop. I hit the gas in neutral going about 65 and still there was no sound. However once I returned the car into gear the humming once again returned!!

Hopefully somone understands what might be going on. Obviously it sounds as if previous posts are correct it being the gear differential is off? Help!!