teetertotter

4.0, Auto: at 50K miles had a whirring sound in rear end. Dealer said it was the rear differential and needed a new kit install. $1500.

At 105,000 miles had the same whirring[whistling] sound and local repair garage with former Ford Mechanic, noticed the sound when test drove my car when had other work on it. New Differential Kit Again, $800

At 127,000 miles, the Whirring[whistling] sound has come back. Does anyone have suggestion? Maybe rear gear oil low and really all it needs is replaced to proper level?

tx_zstang

Yeah...get it fixed by someone who knows wtf they're doing!
You didnt state whether the gears were replaced or upgraded at any time (maybe you dont know or stock gears are being used).
You also didnt mention if there was any sign of leaks at the pinion seal.

If at any time of service, new bearings were not installed or not set up properly, there could be issues with some bearing. Also, if the gears have not been running properly (proper contact mesh pattern), they could have worn wrong and badly.
Was the car ever in an accident? You could also have a bent rear end that is contribution to issues.

It could also be a wheel bearing, depending on exactly what the sound is you're hearing. There are a lot of different strange sounds that can develop, some that'll occur always, some at certain speeds, some on acceleration only, some on deceleration only. All such details are pertinent to proper diagnosis.

If it were my car, I'd replace the gears with a proper install, including wheel bearings and seals. Although, in the hands of someone who knows what to look for, listen for, and feel for, the root cause can be determined. I've worked on a number of these Ford rear ends, often when someone else couldn't fix it or messed it up. Sometimes you can't tell what's going on until you discover it during a rebuild, other times a problem can be found a fixed.

Another option for you, find a local used rear end and replace the whole thing.

It is possible the fluid is low, but unless it is leaking, it wont get low on its own.

teetertotter

The FORD dealer at 50k, installed what they call a Differential Kit.
That solved the Whirring/Whistling sound on acceleration and at traveling at hwy speeds. At 105K miles the same sound returned and mechanic said it needed a Differential Kit. That fixed the sound again until now at 130K miles.

What is a differential kit? No gears were replaced and obviously the differential kit has not solved the problem totally. I would assume that a Differential Kit has bearing replacements.

My car has never been in an accident and new off the lot when bought in April 06.

The whirring/whistling sound is in the early stages and not real loud.
I'll continue to drive it around town. I don't hot rod the car and will let it go for now.

157dB

AMSOIL severe-gear shock proof diff fluid.

http://www.amsoil.com/shop/by-product/gear-lube/