Undulating noise esp. around 40-60 mph (E220CDI S212 auto)

[robaa]

My car makes a rough grinding or whining noise which is at its loudest and becomes 'undulating' around 40-60 mph. This has got gradually worse over the past year or so. I eventually convinced myself that it was one of the rear wheel bearings and had them done... to no avail, of course.

More recently, I've noticed that the sound changes very slightly when I ease my foot off the accelerator even just slightly. This seems to do with whether the car is pulling or coasting, rather than with any change in speed or revs. I'm now worried it might be the rear diff, but I don't know how to diagnose that, and obviously don't want to do what I did with the bearings, ie. get it changed only to find out that it doesn't resolve the issue!

The car drives and otherwise behaves normally. It's 10 years old and has done c.140K miles.

Any ideas what this might be, or how I could get it diagnosed correctly? I realise it's difficult to say without hearing the noise, but any suggestions would be welcome.

Thanks,
Robaa

 

[AMGeed]

Take the car to a Mercedes specialist you can trust to diagnose the issue.
Propshaft bearings could be the problem but its only guessing. Have it looked at by a specialist.

 

[suty455]

swap the tyres front to rear and side to side first, its not uncommon for some tyres to develop a hum as they wear especially All season Tyres which in some cases can seem really extreme, it does sound like a prop bearing though

 

[paulac]

I have the same at about 20-30 mph sounds like a grumbling bearing, had it checked out but nothing found, gut instinct tells me it's gearbox/prop, also got a rotational humming and a lot of tyre noise