A whine on acceleration is almost certainly the pinion bearings. They've probably lost preload due to wear and are physically loose. You may be able to fix it by just tightening the pinion nut slightly. Its not from a lack of friction additive. That would cause it to chatter going around turns when there is a difference in wheel speeds.

 

Ok, so I've heard about this "friction modifier" a few times now SOOOO I went to Ford this morning and bought a 4 oz bottle (less than $10) and just put it in but waiting for the Directv guy to get here so I haven't driven it yet. I'll update you on this tonight. I will assume that as soon as that smelly-*** stuff goes in, the whining will stop? Details to follow!

 

Won't make any difference. If the noise is occurring during straight line acceleration its NOT the differential clutches. You have a bearing issue.

 

X2. I'd say your carrier bearings and or pinion bearing is on their way out. Definitely doesn't sound like a clutch pack issue.

 

[QUOTE=EyeLuvMeyeF150;5010546]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?

For some strange reason it only becomes very audible under load. Have a second person rev the engine while you stand in front and listen to the idler bearings. It'll help eliminate it as the source at the very least.

Good luck!

P.S. I replace all idlers and the the tensioner pulley and the problem was eliminated.

 

Ok, so I drove it last night for quite a while and "PerryB", I think you're right...still roaring. And "Brutus442", I did what you suggested and that noise wasn't audible from the front BUT I will probably still do what you suggested about the idlers and the tensioner pulley anyway and I'll replace the belt because it's never been replaced in the 10.5 years I've owned it. (I know, idiot!)

So, "PerryB" and "screamineagle", is replacing the carrier bearings and pinion bearing something a semi-novice can do? I say "semi-novice" because I have done brakes, rotors, rear shocks (ranchos) and quick lift front struts (ranchos also) and most recently, added friction modifier to the diff! I'm no mechanic but I'm not afraid of screwing with the truck but I have no lift to get under it safely anyway. I got a little nervous with the rear end jacked way up to put the FM in yesterday but used jack stands so it was relatively safe...relatively

 

Wow! Never mind on this 'novice guy' trying this. Just looked at a video on how to change out the pinion bearing. No thanks. I'll take it somewhere for that! Holy crap...