Vibration and hum changing with vehicle speed

[Jacob Castleberry]

My car is a 1997 Mercedes E320 with M104.995 (I6 3.2L) engine and 722.605 transmission. Do NOT tell me my problem is because i drive a Mercedes, and do NOT tell me to take it to a mechanic. I am one.

Now, my issue. My vehicle has a vibration and a hum, as stated before. Idling, my engine is fine and there is no vibration. When driving, there is a vibration that is relative to how much power my engine is making. So 20% throttle = little vibrate, 100% throttle = big vibrate. as i move through the rpm range, the vibration lessens and increases depending on rpm and throttle position.

Now the strange part. When i am traveling at a moderate speed, well say 25 mph, there is a hum. This hum will be louder with higher throttle input, and quieter with lower throttle input. This hum also increases in ‘frequency’ (higher pitch at higher speed, lower at lower) as the vehicle speed increases. Note: when moving at any speed, removing all throttle input completely eliminates the hum and vibration.

I replaced some components on my engine due to this issue, though the components i replaced were cheap parts i put on for repairs when i was out of $ and figured id put some good name brand in their place. This includes: Spark plugs/cables/boots, ignition coils, and i replaced the factory EO harmonic balancer with another (used) OE harmonic balancer. I did some other unrelated maintenance/repairs during this time due to “already being in there”.

My belief is that my transmission is at fault here….. I really really hope not, but i think that may be the issue. im also thinking maybe my driveshaft u-joint? or possible my rear axles or rear dif? I cant just go replacing it all, so i need to know how tf to diagnose this issue. it only happens when driving, and at moderate speed. i cant get under the vehicle during that time.. sadly..

Any help is appreciated. Thanks in advance.

EDIT: I just test drove it again, and i think its the driveshaft carrier bearing (?). The sound seems to be coming from that location, as far as i can tell.

UPDATE: after further research, my symptoms seem to point towards failed differential. Sound/vibrate under accel, no sound/vibrate under decel.

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