2004 FX4 has a humming sound sort of like mud tires on highway when I accelerate a little or a lot. When I let off the accelerator, it stops. It happens wether the truck is cold or been driving all day. Seems to be when cruising and holding the accelerator at a steady speed. When i let off, it stops. When I press on it, it Humms. It seems to be coming from behind me or directly below me. I was thinking fuel pump??? It just drives me crazy. What do you think?

 

Sounds like U-joints or rear

 

Why would pressing the accelerator make the U Joints make noise? I do not know anything about this so excuse the question if it sounds dumb...LOL
It seems like it would make the noise all the time?

 

My truck has been doing the same thing since I bought it a year ago. Hasn't gotten any worse or better. I've checked the u-joints, changed the diff fluids, transmission oil, and transfer case oil and its still doing the same thing. Its strange how it only does with the slightest acceleration or when cruising. As soon as a let off the pedal at all it stops, then comes back as soon as i start to accelerate. I'm also stumped as to what is causing it?

 

ssrider...I wish I could get an answer to this problem. It is very annoying...LOL
It's not causing any problems but like you said, it makes the noise with only the slightest pressure on the gas pedal...
Anyone know what can cause this?

 

My rear makes a "whine" sometimes when I accelarate. It is kinda high pitched. I think there is a TSB for it. Not sure though.

 

What is a TSB ?

 

Technical Service Bulletin

https://www.ford-trucks.com/tsb/tsb.php

 

Maybe this is the TSB. It talks about the humming noise on acceleration.

2005 FORD F150 TSB's (technical service bulletins)

 

i also have a slight hum, sort of like the way aggessive tires sound, it does it exactly the same way you guys have it. Mine has been doing it for the 3 years ive had my truck, put 30,000 miles on it, never got any better or worse. And like other have said, nothing seems out of the ordinary, i just turn up the radio.

 

I have seen when the gears in the rear end get worn they will make that noise only under acceleration. It doesn't mean your rear end is worn to the point of failure but that they could have a wear pattern that causes a whine under acceleration. Really no way to fix with out replacing them. Bearings in the rearend or output shaft in the transfercase can cause the same issue. Kind of a tough issue to isolate.

 

I'm thinking it could be the output shaft on the transfercase because it almost sounds like its coming from under the truck. Any idea how hard the bearing is to change?

 

This sounds like a pinion bearing or a gear wear pattern issue, not likely a t-case bearing as they are not a tapered roller so it will not be affected by a load change (noise would be constant). It is possible that a siezed ujoint can apply undesired load on the pinion creating a change in pinion to crown gear contact resulting in a noise, but it would have to be severe and the pinion bearings are likely starting to fail at that point as well. Throughout my years in the industry 9 out of 10 will be a differential noise if it comes and goes with throttle application.

 

DO you have Limited slip? Mine was doing the same thing, and it ended up being my Limited slip packings being shot. And after 2 sets of Clutch packings. The local Off-road drive line shop said to drain SYN 75-140 out out and get 3 bottles of FORD Additive and run NON SYN 80-90 with the 3 bottles of the Additive. They said that the Synthetic fluid Kills the packing. After taking their advise I Have eliminated the Howling noise and the rumble of my clutch packings...

 

Ah-ha (LSD)... Thanks! Same noise in my 2004 F150 5.4 FX4... Mine sounded like a bad tire and/or a fuel pump buzz as well but it must be the limited slip rear-end after reading all of this.