87 300d 178,000 miles

Whining noise from rear end. I thought it's tire whine, local mechanic after driving it says "rear wheel bearings" and suggests replacing both sides.

It's hard to pinpoint the source of the sound, but there is no thumping or bumping, just fairly high pitched sound that is annoying. Doesn't seem louder at higher speed, starts at about 35 mph.

Any suggestions greatly appreciated. Does symptom indicate bearing? Could it be tires? Is there good diagnostic method to improve chances of finding if this is the correct solution?