I had a 300SDL that did the same kind of thing you describe..

Driving straight on the highway there was a noise, but it went away when you went around corners in one direction. Straight away and the other direction produced a howl.

Here's what I tried (in order)

-Looked at axles and differential and boots.. all looked good
-Had MB tech look at it, he said "wait till whatever it is breaks, then we'll replace" as he couldn't find anything wrong
-Drove 3 more months and the sound began to get on my nerves.. took it back to him.
-He replaced left wheel bearing and said the old one looked "almost brand new" but that was his best guess. Problem still there

2 months later I decided to change the differential oil, and it TOTALLY fixed the problem! Waited 2 weeks and flushed the diff again and filled up with new oil, just to be safe. Drove 35,000 miles (approx) and the sound never came back.

Before looking at wheel bearings/etc, are you sure your differential oil has been changed?

__________________
'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)