1992 190E humming/vibration at highway speeds
 

My 190E is not quite the smooth highway cruiser it used to be. There is a hum at highway speeds. It sounds like someone blowing air around the top of a beer bottle (like a flute). From my experience, the sound from a wheel bearing is slightly lower pitched. It is not a constant hum either. Even if you maintain a steady speed, the hum has volume fluctuations. It starts at around 100 km/h and at 110 km/h the noise starts to get less loud. At 120, you can still hear it a bit but not as much as 100. There is also a vibration that seems to appear at 130 kmh. The car used to be able to cruise at higher speeds with absolutely 0 vibrations.

Also note that:

-I replaced the 2 front wheel bearings. One was shot. The noise remains.
-The 2 rear wheel bearings don't have any play and don't seem to get hot after driving for a while.
-I tried to change the diff oil but the drain plug is stuck! I tried the impact on it and it won't bulge! But when you open the fill plug, oil comes out. So at least it has some oil in it...
-I also changed the driveshaft bearing and the carrier. The bearing was so-so but the rubber carrier was blown. The noise is still there. BUT I might have put the driveshaft back on with one spline off... I think I did it right, but not absolutely certain.

I plan on dropping the rear subframe when I put the car in storage in a month. Do the bearings, bushings, diff r&r, etc...

speedo cable?
 

A speedo cable that's faulty can cause a hum and vibration. There no cure for it other than replacing it. it a relatively cheap and easy DIY fix.

Really? Actually at certain speeds the gauge's needle bounces up and down. Where does it plug in? At the transmission?