1993 Mercedes Benz 300E, right rear squeak

Common causes for High-pitched noise from rear wheels are:

The wheel bearing: this is not usually dangerous in the short term, but I recommend contacting a mechanic and having it fixed. To test the wheel bearing, you lock front wheels, and jack up the car rear and spin the wheel (handbrake unlocked). If bearing is worn you hear a drone. When is the bearing, often car starts vibration above 40 mph.
Noise from bearing can be continuous, intermittent, only when steering, only when hot.

The brake pads: when brake pads are worn out they start making a squealing noise, in this case the noise is only when you brake or continuous. When you jack up the car the wheel may be a little stiff, but not necessarily.When brake pads are completely gone you also hear a scratching noise when braking. Inspect your brakes immediately.

Other suspension problems: Jack up the car as suggested above, check if the wheel has a slight play on the sides. Check all of the suspension components, suspension mounting points, and steering linkage for damage. Also, check the frame rail for wrinkles. It is very important that these components be in good condition, if you notice any problem here, go immediately to a mechanic.

Try doing the simple test above, reinstalling the wheel and working the tire back and forth. This is a common test for bad ball joints and wheel bearings.