So, when I'm going down the road, whether it's 10 mph or 80 mph, there is this annoying whine I can hear. Passengers in my truck complain about it too. It only does it when I'm accelerating. I thought it might be the fuel pump because when I let off the gas, it stops. But as soon as I push it, even barely, it starts that crap again. I've heard it could be rear end gears wearing out but I had that serviced a couple of years ago and had that specifically inspected for wear. (The whine was happening before and after that service too). I've heard alternator whine but it really sounds like it's coming from the back of the truck so I'm more inclined to believe the fuel pump or rear end is to blame. Any help would be greatly appreciated.

 

I know you mentioned it seems to come from the back of the truck, but I experienced the same conditions you are and it turned out to be the idler's bearings failing.

After replacement, the sound dissappeared.

I hope you locate it, because I share your frustration with noises and explaining it to passengers.

 

At that many miles, if you haven't properly maintained your diff, I'd be inclined to start looking there.

 

Brutus442: If it were the idler bearing (idler pulley?...novice here...) wouldn't it do it all the time and not just while I'm accelerating?
And...
nriver04: Same thing with the differential. I would think it would be a constant whine if it were that but I'm also a novice at these things.
In doing more reading on this site, I'm seeing that the idler pulley and tensioner pulley probably need to be replaced and I might as well replace the belt seeing as how I have never replaced it in the 10.5 years I've owned it and probably never was changed in it's first 2 years...

 

Do you happen to notice the whine while the hood is open? Have you tried having someone rev the engine while you listen? It sounds to me like a pulley issue, as you just stated...

 

I had an idler pulley whine like that on my motor.. almost sounded like a turbo spinning up..

 

So I was telling a guy about this whine I have. Being a mechanic and owning his own shop, he thinks it's the rear end. He asked me about when I got it serviced. I looked back and it was actually 2011. He then asked if an 'additive' was added to the rear end fluid that was changed. I had no clue. Has anybody ever heard of that? He told me that if this particular additive had not been put in, that would cause it to whine.

 

The additive he's talking about is a friction modifier used in limited slip differentials. It's required when changing your differential fluid. Most differential fluids come with the additive included, but I've read that this is largely ineffective, and a separate bottle should be added. I just changed the fluid in my differential about a month ago, and added a separate bottle. It would be very easy to put a bottle in if it's something you'd like to test. I bought mine off amazon for 8 bucks.

 

I just bought an 04 f150 xlt with 4.6 v8. From the time I bought it I have had the same whining? I'll mention the above comments when I have my mechanic look at it.

 

Sometimes the foam seal around the air filter goes bad. Try removing the air filter completely and go for a test drive.