My 1998 Explorer 4WD has a hum that is coming from the rear end. The hum is simultaneous with pressing on the accelerator. When my foot is not on the accelerator it goes away. When i press the accelerator it comes back.

It seems like this hum developed after I had the fluid in the tranfer case replace. I had it replaced again (for free) for fear that they used the wrong fluid and it still makes the noise.

Pulsing the accelerator such that I basically maintain the same speed (70 mph for example), i can hear the hum come and go with each pulse.

It could be something with my muffler but I did not see anything obvious.

Thanks for any help

 

Wife's 99 Mountaineer does the same thing. Guy at work had two company explorers. Both did the same thing.
I think it's the 3.73 truck gears and is inherent by design. Only difference is in a truck the rear axle is outside of the cab and you can't hear it.
Sound far fetched?