93 F-150 5.8, auto, 4x4, super cab.

I need some help. I have been trying to figure out what this sound could be and I am at a loss. Here is the deal:

The sound is directly related to the speed of the truck. When I am at about 25 mph, it is a low hum with minor vibration, when I hit about 40 mph, the pitch is higher and the volume is definitely the highest of any speed. As I speed up on the freeway, the pitch gets so high that you can barely hear it any more. If I let off the gas at any point, the sound goes away and the vibration stops. The vibration is hardly noticable, but is present when the noise is there. The noise seems to have a somewhat metallic sound to it, especially at higher speeds.
Like I said, it is directly related to speed and throttle. If I am not pushing on the gas, there is no sound or vibration. It is definitely not the tires and it something mechanical, but I cannot figure out what it is. I changed the diff fluid and transmission fluid. There are no leaks coming for the pinion seal so I don't think it is an issue of pinion/ring gear alignment.

What is spinning when the drivetrain is under load (giving it gas) but is not spinning when you let off the throttle? This should hopefully help narrow down the search. Any help is greatly appreciated because I am about to lose my mind.

 

Sounds like its engine related..maybe a bearing? motor mount?

 

If it were engine related, I would expect it to be directly related to rpm, not mph. This makes me think it is between the transmission and the rear wheels, but I'm just not sure.

 

My truck does the same thing and its the rear end.

 

if its the rear end someone did not put the shims in properly you will need to get that fixed by a pro if you try to fix it yourself it will get worse unless you are just really lucky. it is very common for all rear ends on most cars trucks and vans when someone trys to re-gear it or just need to replace something they don't take all the steps to make it right.

 

I have kept mine how it is because the sound doesn't bother me.

 

Anyone know what shop typically charge for that? I wouldn't mind except for the metallic sound. With how fast things are spinning, all that sound says to me is that metal is wearing down faster than it should.