Highway 'humming' noise
 

My 2006 E55 has developed a 'hum' which is very noticeable when on the highway. It is not tyre noise and it only varies with speed, not with on/off throttle so is not the diff and shifting to a lower gear does not alter the noise.. I suspect a wheel hub bearing(s) or tranmission output shaft bearing. The car has 110k km only. Would either of these issues be common on these cars? Cheers.

Wheel bearing is the first suspect. Can you sit in the back seat with someone else driving and try to find which side the noise is coming from?

Put the car into a slow left and right hand turn....like a slow bend in the road or just turn the wheel left and right as you are going straight. If the noise gets more quiet during a left turn you have a bad bearing on the left side and vise versa. all you are doing is taking the load of a bearing and the noise will change.

Can you be a bit more descriptive, I'm not quite following: "...it only varies with speed, not with on/off throttle". Since you must engage the throttle to experience speed variations, the statement is a bit misleading. Where I'm going with this is... it might be your diff. you shouldn't expect a dramatic change upon engaging the throttle, BUT, you will experience a change in the sound depending on the speed. I'm not trying to be an alarmist, because it very well could be your wheel bearings. I would still suggest that you get the car in the air, have an indie shop that you trust put a stethoscope on the diff under acceleration. Eliminate that from the equation and also look at the wheel bearings.

There is also a carrier bearing on the drive shaft.

I have had bad diffs before and this is not the same. A bad cwp will change tone instantly when the load changes direction when you on/off the throttle. The noise is directly related to speed no matter what the inputs. Only the drive shaft, half shafts and wheels have a single relationship to speed. This and the other replies confirm in my mind that it can only be a wheel bearing or the output shaft bearing in the trans or the drive shaft carrier bearing. Thankyou all for your help.

I have determined that the carrier bearing for the drive shaft is at fault. There is a bit of play when pushing the drive shaft up and down on the hoist. There seem to be many of these for sale on Ebay. Have any of you replaced this without going to the Merc dealer and can recommend a supplier? Cheers.

FCP and give them your vin. There are 2 sizes of bearing and the larger one my be hard to get. There was a thread about this maybe 5 years ago. If you can afford it, this will be a great opportunity to replace your flex discs too.

Thanks but who are "FCP"?

FCPeuro.com. A parts site for euro cars. They have a free lifetime replacement guarantee for almost all parts for as long as you own the vehicle. It includes brakes, filters, etc even oil. Only exception is consumables that cannot be returned (i.e. sprays, etc) :y

I changed the driveshaft carrier bearing with no change in the noise. It is definitely not a cwp issue as the noise is the same on or off the throttle. I have swerved left and right with no sound change as well as checking all wheels for play on the hoist so not a wheel bearing. That leaves the trans output shaft bearing or the diff input shaft bearing. The noise is still there when I put it into neutral at speed so does that eliminate the trans bearing? Thanks in advance.

Is your car lowered?

Car is not lowered. Definitely not tyre noise as I have purposely driven over changed bitumen surfaces and the tyre noise changes but the pulsating hum does not.

Follow up to transmission output bearing
 

Did you end up changing the output bearing? I’m looking for information on how to replace the output bearing on my E-350 rwd and seem to only come up with AWD info.

It was the tyres. Sorry but I was well and truly fooled.