Chasing Vibration, W210

[gmog220d]

1996 E300, 190K miles

So I have this humming, droning, throbbing vibration that starts around 60 MPH and stays at it up to 85, which as fast as I have driven it. The frequency changes with speed. I’ve been focused on the drive line, but all the changes I made have not made a difference. I’m beginning to suspect a rear wheel bearing assembly is to blame, but when I check them with the wheels off the ground they feel fine - no play, no rumbles or notchiness.

I can feel it through my feet on the floor, my butt in the seat, through the steering wheel, and through the console around the shifter. It’s subtle, though. My gut leans towards the sound coming from the rear end. Swerving right and left doesn’t make a noticeable difference.

I’ve dug around looking into both drive line and wheel bearing threads but they are all over the place in terms of symptoms and fixes. What’s worked for some has not worked for me. Here’s what I’ve done to the vehicle in the last year or so:

- Rebuilt rear suspension with new links, new lower control arms, new springs, and new carrier bushings;
- Fresh tires;
- New Bilsteins all ‘round;
- Rebuilt, balanced driveshaft with new u-joint, center support bearing and bracket, and fresh flex discs;
- Welded in front spring perches and rebuilt the front suspension while at it;
- Wheel alignments after each axle rebuild;
- New wheel bearings on front end, setup using a proper metric dial indicator;
- New engine and transmission mounts (installed last weekend); and
- Adjusted drive line angles by adding/removing shims at the trans mount, center support, and diff, and also by installing the fresh mounts.

So what the heck does W210 drive line vibration feel like compared to rear wheel bearing related vibrations, or even front wheel bearing vibrations? Someone out there experienced and fixed both in the same car?

And if you’re still reading you must really be interested . . . .

I found documentation on what drive line angles are supposed to be for W210 chassis variants (see attached pics). I took measurements where I could, between the trans and front section of the drive shaft, and between the front and rear sections of the drive shaft. I can’t find a surface on the diff that provides consistent measurements.

To get the angle of the transmission I measured off the bottom of the steel pan. I found multiple locations to read from that gave consistent readings. I used a magnetic digital angle finder that’s pretty sensitive. I made repeated measurements and averaged the results. I found that the transmission seems to sag towards the front, creating an out-of-spec angle between it and the front shaft. I hoped replacing the mounts would fix that but I only got a half degree out of it, leaving me with a measured angle of about 2 degrees + (should me more or less 0). Is it possible the steel pan is not level with the angle of the transmission, thus giving me a false reading?

Pull the drive shaft and have it checked out? Go ahead and replace the rear wheel bearings?