My son drives my old Merc to work. He reported a sound at highway speeds, so we took a drive today. This doesn't seem to happen around town. Only at highway speeds. I'm kind of assuming it is attempting to shift from 4 to 5 or vice versa. I didn't count gear shifts heading out. I guess I should.

It almost sounds like rumble strips. Kind of hard to hear in the video. A little less so in real life, though not startlingly loud by any means. I assume it's not the diff or CV joints on the drive line as it only happens at high speeds. I believe it is a 722.623 transmission. I did change the (lifetime ) fluid in it when I got it five years ago.

Any thoughts appreciated.

Harmonic vibration?
Driveshaft rag coupling/flex joint.
At the diff.

quote:

Originally posted by ScreamingCockatoo:
Harmonic vibration?
Driveshaft rag coupling/flex joint.
At the diff.

I like to hear possibilities that aren't tearing out the transmission! I did have the flex disc changed preventatively when I bought it. But no other components.

From an old thread on an MB forum, one guy described the problem pretty accurately, and said it self resolved: "The humming noise went away after my right rear cat started rattling and I replaced the cat. I'm thinking that it was the matrix in the cat coming loose and vibrating against the housing at certain engine speeds. Once the matrix broke loose, the vibration noise went away. I'm now at 148K miles and I'm starting to hear it again so I'm waiting to see if the left rear cat starts rattling soon. "

Also the alternator pulley clutch can be going out.

As a formulator at an additive company, with positions at all major OEMs, may I offer these thoughts:
1. Shift Clutch Noise Vibration and Harshness (NVH) is not continuous.
2. Shift clutch NVH can actually be other transmission system issues, such as a failed accumulator.
3. I don't think your car has a torque converter clutch, but if it does, NVH with that large clutch is possible. This NVH would be a the vehicle speed and transmission gear where the TCC is locked up. It would only oscillate if you were magically driving at that decision point where TCC should lock, or maybe be open, or maybe lock, or well, we don't know what r0gue really wants...
4. Continuous NVH at a single speed is likely resonance or shaft alignment issue.
5. Changing the lifetime fluid was a good idea, chemically. The fluid is designed to be lifetime, and tested within an inch of its life, and yet chemically the friction controlling components do slowly degrade.

quote:

Originally posted by Mars_Attacks:
Also the alternator pulley clutch can be going out.

The sound is definitely in the rear, or maybe in the middle at the source, but being audible routed through the rear.