maybe someone can shed some light on this.
 

I have been hearing a strange frequency when I am going above 60 mph all the to 80 mph, I will try to explain the best I can.

I thought it was my exhaust that is causing the problem, but then I noticed the sound only appears when I am crushing or when I accelerate from 60mph to 80mph, basically instead of a constant (linear?) "exhaust" tone, it gets wavy. Wavy like it would go from low to high and low again. could be the engine or the exhaust but I can't pin point what caused it.

it wasn't like this before so any help or info is greatly appreciated.

Plz provide a video or sound clip, meanwhile get under the car when its cooled off and try to see if anything is missing screws anywhere or they are not tight or so, specifically, but not limited to drive shaft, half shafts, engine and transmission mounts, exhaust mounts, anything that could rub against something. Something leaking and so on.

Pull on things see if it moves at all or too much example. the drive shaft on manual transmision will twist about 1/2 in left right, over that distance is noo good. check if ur differential bushing is leaking.

If you have an OB2 scanner or w/e, to pull codes n see if anything is wrong with the engine it self.

The "wavy" effect is probably harmonics - two or more pieces are vibrating at close to the same frequency. First thing I'd check is the balance and tire condition on all four wheels.