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My car is a 1997 E320 with the M104.995 (I6 3.2L) engine and 722.605 transmission.

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 possibly 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.
 

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You didn't state the mileage on the vehicle.

Motor mounts, tranny mount, flex discs, carrier bearing, for sure. I'm not exactly familiar with your model, but in a general way you are on the right track.

Don't let this get to the point where this stuff comes apart at speed, it makes a mess under there from what I understand.
 

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Discussion Starter #4
You didn't state the mileage on the vehicle.

Motor mounts, tranny mount, flex discs, carrier bearing, for sure. I'm not exactly familiar with your model, but in a general way you are on the right track.

Don't let this get to the point where this stuff comes apart at speed, it makes a mess under there from what I understand.
Ive looked at alot of the drivetrain mounts, and they all seem to be firm and secure. After alot of research, im now beginning to think my issue is with my differential. accelleration causes noise and vibration, under coasting and deceleration the noise and vibration go away. from what ive studied so far, this says differential.

What do you know about differential failure?
 

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Diff noise = whining gears that increases as speed increases.

Vibration is likely a driveshaft issue such as the front and rear flex discs, center bearing or unbalanced driveshaft

Try changing the diff fluid, and see if anything changes. If not, it'll only get more expensive and difficult.
 

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Changing out harmonic balancers isn't an everyday DIY project. Then to replace with a used unit just adds more "unknowns" to the issue at hand.

How did you know the original balancer was bad? When the car is parked and running, how many of these shakes, hums and shimmies show themselves?

Jayare

Sent from my XT1710-02 using Tapatalk
 

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What happens if you put your car in neutral while it's going 30mph?

I would change the flex disks period, unless you know they are good. And look at the diff mounts, driveshaft carrier as instructed.
 

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I would check engine mounts first to make sure the sump is not hitting the crossmember when the engine tilts when being put into drive. Costs nothing to check.
I previously got a vehicle very cheap (stupid cheap) because of this, the owner thought the auto box was faulty.
 
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