Friend's 92 400E (W124), 79kmi, has a high-pitched squeal, not very loud, when rolling along slowly; 'sounds like it's from right, rear. Squeal does not change with changes in engine speed, nor when brakes are aplied. Seems to be at wheel, axle, driveshaft rotation rate. Squeal goes away as speed increases, then comes back as lower pitch - not so much a squeal, but as a whine. Any ideas, tips? TIA