HELP - Strange RPM Related Noise

[Dervman]

My SDL has developed a strange 'drone' noise but it is intermittant and so far has been difficult to pinpoint the source. It feels as if the drivetrain is the source but I can't be sure.

It only occurs after the car is fully warmed up and usually noticable after a highway run. It seems to happen in each gear between 2250 & 3500 rpm when the car is under load and can be felt as well as heard as a vibration. If I back out of the accelerator, it goes away. I can't find anything obvious that is loose. Temperatures & pressures are fine. Tranny fluid level is normal and clean. It cannot be reproduced at idle or by holding the car on the brakes in gear and reving the engine. HELP

[billrei]

Everything feels OK from where I'm sitting.....Try it again. If you think diagnosing noises while riding in your car is tough try it from here....

Might be something in the drivetrain maybe a failing flexcoupler?? Try turning up the radio until the problem worsens or something falls off....

[TX76513]

Going to toss my "weird noise" out that I fought for some time - at least you may be able to eliminate something. Had similar dull noise, only at +45 mph. Chased this for months. On the windshield a piece of the rubber gasket came loose. (Looked like a strand of rubber band). At +45mph the wind would pull from windshield frame and it would slap body of car. Slow down it would retract back to frame.

Crazy but true

.02

[Dervman]

In my mind I had already eliminated anything that was srictly Road Speed related as it seems to be RPM related under load, but only when warm.

Could bad motor/tranny mounts cause a low frequency vibration/drone like this? They 'look' OK but they are on my "to-do" list since I have no idea if they were ever replaced yet. The sound is bad enough that I'm really not happy about driving too far.

[soydrivermatt]

Have you tried lightly applying the brakes with your left foot while keeping it going with the right? Might help you decide if its brake related....

[Dervman]

Noticed on the way home that my RPM's were much higher than usual. On the highway I was turning 3000rpm at 75mph. normally its about 2000rpm.
Then I checked how many gear changes I had from a dead stop and it looks like the tranny isn't shifting into 4th gear
Any ideas where to start

[suliot1]

Don't know which car you have, but just over 3000 is normal for my 300sd in fourth. Never had a gasser that would do 2000 at 75, let alone a diesel. How's your tach doing?

[Brian Carlton]

Andy,

The SDL normally turns about 3100 at 75 mph. It never could have run anywhere near 2000 at that speed.

Vibration issues are extremely tricky to diagnose in person, much less on the screen. We likely would be shooting in the dark unless someone had the exact same issue.

[Dervman]

I guess I must have been dreaming about the speed/rpm
My only excuse is I was born at a very early age
Nice to know my Tranny seems OK though even if I can't count gear changes

I'll have to dig a little deeper and see if I can pinpoint the source of the noise.
I'm still wondering if bad mounts might be the cause.
Thanks for the input guys.