whining noise from rear that increases with vehicle speed, not RPM
 

It sounds like a jet engine, but not as loud. Very high-pitched. anyone know what this could be?

Differential
 

possible low oil, suggest a change to synthetic.

Yep........what he said.........

If that doesn't work:
 

Give that 3 year-old in the car seat a Tootsie Pop and see if that attenuates the whining for a while.

:)

Sorry. That was a parenthood-induced reflex.

you guys mean low diff oil? I would think it was wheel bearings.

I'd say it's a wheel bearing too. I had that happen on a front wheel a few years ago & that's exactly what it sounded like.

Put the wife in the trunk and drive around. She should be able to tell you if it is coming from the center or one of the wheels.

Jack the rear up and see if their is any play in the bearings. That would tell you pretty fast what it is.

Hello HIDGolf, McRoth and Hatterasguy
 

Rear end whining in high frequency is differential 99.999% sure. :(
Bearings are a grinding, rumble, growl or howl. :eek:

Checking the bearing play will hurt nothing and may find other issues before they are serious. :)

thanks. i'll change it tonight...i'm going to give synthetic a try. i've never changed diff oil before...what kind of oil do i put in there?

I don't remember the size but you will need a big allen wrench. Also take out the top plug first!!! If the top will not come out stop; because if you drain the fluid and the top plug wont come out you now have a bigger problem.

I don't know the capacity on the W123 but on my W126 is is 1.2 quarts if I remember correctly. The manual should say in the back.

I recommend Mobil 1 75W-90, autozone has it for $8 a quart last I checked. Cheap insurance.

Whunter how often are you supposed to change the diff fluid? I will probably do mine every time I change the trans fluid. So every 30k?

The determination of "differential" or "wheel bearing" can be assisted if the following question can be answered:

Does the noise almost go away entirely when the drive shaft is unloaded?

The drive shaft is unloaded when you are cruising along at a steady speed and you lift your right foot ever so slightly so that the vehicle just begins to slow down. If you lift too far, the shaft will load up in the opposite direction and the noise may return.

Please attempt to reduce or eliminate the noise via your right foot, in the manner described, and report back.

Personal opinion
 

Every 100K miles with synthetic.
That is how often I change mine.
I buy a used vehicle, change all fluids as a base line.

Lately my car has had a high pitched squeal from the back too, generally only at lower speeds....(20-30mph) and I swear its coming from the right rear wheel, perhaps a dragging caliper? it doesn't always happen either, only occasionally. But it does sound like squeaky brakes. I haven't noticed any difference in wear of the brakes on that wheel though. I am paranoid about my diff. fluid now, I will be changing it when I do my oil change in a few weeks....I doubt its been changed in ages, so its a good idea.

Caliper or parking brake corrosion
 

would be my best guess on your squeal.
Remove rotor and check parking brake shoe and inside rotor for heavy corrosion.