My 1996 850 with 170,000 miles has developed an intermittent, but more frequent, squealing sound that begins above 10 mph and gets higher in sound until unoticeable at highway speeds. It stops immediately when the car stops moving. No steering, braking, or coolant problems at this time.
Any ideas?

I would have your parking prakes checked, the shoes tend to fall apart and rest on the drum.
Drive around the neighborhood with all of your windows rolled down and see if you can isolate the noise.

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

I had a similar problem, but it wasn't squealing, more like a loud metal rubbing sound. I changed the rotors and brakes pads, the noise is gone. don't know if its the same situation or not, but check your pads/rotors for wear and rust

Does the sound change when you accelerate or coast, or when you turn left or right? Often - NOT always - a constant unchanging sound is a wheel bearing while one that varies with torque and load could be a drive shaft (CV joint).