If I'm just kinda slowly cruising along at about 25mph in my '84 SD, I can hear a kind of a low growl coming from the rear. It's not real loud, and after putting about 15,000 miles on the car since I bought it, it doesn't seem to be getting any louder. When I took the car into a repair shop a couple months ago to have the cruise control fixed, I asked them to check it out. I thought maybe it was a wheel bearing. It seems to be coming from the left rear , but that may be just where the sound is able to resonate up thru all the seats, insulation and carpets. Anyhow, they said everything checked out fine.
Is this just a normal sound coming from all the CV joints ect, or from a third member that has over 300,000 miles on it? Any ideas out there?

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past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)