No input at all??

I was driving home last night and the highway was empty, so I played around a bit. Determinded that the humm occurs ONLY between 125-135 km/h....going faster than 135the noise goes away. It is loudest right at 130km/h.

Any thoughts?

 

I was going to say tires but just read you tried that already.

I had this happen with some cheap tires.
Actually they drove well, rode well, were on track to last a good long time,
but they made some much noise I ditched 'em.
They were Federal brand tires.

I thought it was my diff, or a wheel bearing.
My indie took a ride with me, and said it was the tires.
I found out he was right when I switched a different set of wheels with different tires.

 

Yeah, thanks for the input, but I've tried a few different sets of tires already. No change. Also, the fact that the noise goes away the second I lift off the pedal (or accelerate past 135) leads me to believe that it's gotta be something driveline related?

 

I had a sound like that on my SLK, turns out to be the rotors. Brake pads previously damaged rotors and made their surface uneven. New brake pads didn't fix the problem because rotors carved pads to the shape that kept irregularities the way they were. I had to change it all. And then after spending money a week later I had the accident that totalled my car

 

Sorry to hear that Thanks for the input....Rotors, although weird, maybe a possibility here, mine are getting a little thin.

 

So, to bring this back from the dead.

I got underneath and checked the diff fluid, still right up and nice and clean (no signs of any metal in it). While I was underneath I did an inspection myself for anything the dealer may have missed, but everything looked in great shape.

I talked to a couple non-mercedes techs who believe it could be a wheel bearing issue. Apparently, they've seen cases of wheel bearings only making noise at certain speeds.

Anyone else think it could be a wheel bearing? What is the best way to check them?

 

The hum you are hearing is the being generated by the pinion gear in the rear differential. I had the same issue. MB split the R/R of the rear diff with me. It was better but not gone.

In the SLK you are basically sitting on the diff. I spoke with my indy. He and I gazed out at all the MB cars parked in his lot and said to me that a very high percentage of those parked cars hum. In sedans the diff is far away. Maybe some rubber/tar sound proofing may help. I suggested that but was told not to waste my money.

Zill.

 

Thanks Zill!

That's in line with my original expectations. I guess you're lucky MB split the cost with you, I know my dealer certainly would not. If you don't mind me asking what was the total bill for your job?

It does seem like the noise echoes into the cab, I cannot hear it if the window or top is open, but with everything shut it can be quite loud in the cabin.

Would I be doing any damage by continuing to drive?

I too thought about some sound deadening, maybe some dynamat on the bottom of the removable trunk floor?

 

It was established some time ago that Dynamat or similar on the bulkhead behind the seats and the door lining is the most effective way of reducing road noise in the SLK.

Some have lined everything possible, including the panels around the roll bars.(there are good pics - search 'dynamat')

The trunk will transmit noise through the gap in the flaps that open for the vario roof, so insulating the trunk can help (not a problem with normal sedans)
I insulated the the bottom with 1/2 inch sound foam and carpet/ the trunk lid with 1/2 inch foam and frontrunner and the sloping panel above the roller blind (just pull out the carpeting and stick on) - made a difference.

Also put some additive in the new diff oil that had a soft metal component.

Oberoi

 

This is a long shot... What is the condition of the motor mounts? If they are severely worn, the engine will shift position a bit between applying the accelerator pedal and releasing it. This will cause a slight shift in the alignment of the drive shaft going into the rear axle and might cause a change in noise or vibration. Mine were worn pretty bad when I replaced them and a lot of minor, and not so minor, vibrations disappeared and overall noise decreased. Again this is likely a long shot for you.

 

Wow, great job on the sound proofing. I am definitely going to look into something like that.

Someone else actually suggested the engine mounts (or transmission mount) I checked them out and they're in good shape, recently replaced actually.

 

I also think it is a wheel bearing, put on rack and test with no load on wheels, if it's either bearing or rear end should be able to hear it.

 

Nope, sorry guys. This one is pretty straightforward. My motor mounts were fine. The harmonic hum sets up loudest between 75 and 80mph. Take foot off accelerator pedal and the hum just about disappears. The slightest touch on the pedal and it returns. That is the pinion gear in the diff. Not a bearing. Cost to replace ~$1,700.00. Only reman differentials are available. When I had mine replaced the service manager readily acknowledged the hum and said that while MB would replace he could not guarantee it would disappear. It lessened but did not totally go away.

Stay away from that speed sweet spot, use the audio mechanic and put the windows down.


Zill.

 

Bazzle posted a bit on analyzing diff noises some years ago - quite informative and of a general nature that will indicate exactly what part is worn

Here it is - http://www.4x4now.com/gt0699.htm - Thanks to 142tpl not Bazzle

Oberoi

 

I know this is a necro bump, but did anybody find out if the problem was solved by replacing the pinion gears? I have the same thing pop up and oddly enough I suspect the super charger as it blows. Through the system.

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