My strong bet is on CV Joint. Sounds exactly like the noise my 300E made. It increases in frequency with the speed of the car. The other big clue is that reverse will stop the noise. This is a well known trick with bad CV Joint.

If you look around the rubber boot, it will probably be split and greasy. Mine made noise only in cold weather and went away after several miles. Then it got worse and finally, one fine day close to home on a city street, it started an ungodly screeching. Made it home at 10 mph, replaced the axle with a new one and problem solved.

Steve