Humming sound from rear

I have a 1998 Ford Explorer XLT whenever I step on the gas peddle I get this humming sound coming from the rear of the truck. It only does it when I step on the gas peddle and stops when I lift my foot off the peddle. Does anyone know what this could possibly be? I keep thinking fuel pump. It seems to have started a couple months after I had the transmission replaced.

I have a 2004 Explorer and just had to have the rear differential replaced. between 40 and 50 when I pressed the gas it would humm and whe I got to 50 or took off the gas it would stop. I see you left this post last month, hopefully you already figured it out.

I have a 2004 Explorer and just had to have the rear differential replaced. between 40 and 50 when I pressed the gas it would humm and whe I got to 50 or took off the gas it would stop. I see you left this post last month, hopefully you already figured it out.

florit04, I hope you are still answering this thread since it has been posted last NOV. I have the same problem with my 03 expolrer. Humming from the rear diff on speeds between 50 and 65Mph only when I apply throttle. Humming is not very lous but it is there.
I am wondering if you had loud humming or was it just something that ticked you off. I've had that since I got the car and I am wondering if it is a big thing to change the diff. How long did it take? what do you advise?

thanks.

The problem was in the rear differential. I just got your post today at work, so I will go look at the repair bill tonight and tell you what they replaced. Once it was fixed it has been fine.