Noise from back wheels
 

ML350 2003 (33,000 miles). I wonder if anyone can help. There is a whining metalic noise from the back wheels when the car is in motion at low to average speed. The noise stops when the brakes are applied. New brake pads were fitted all round seven months ago and the discs also appear ok, no scratches or scoring. No warning lights on the dash.
I also noticed that the noise is less frequent when the weather is cold.

I was going to say wheel bearing (I just replaced one) but at 33k miles they should be as brand new at this millage unless you do a lot of off-road driving.
I would suggest jacking up your car and rotating wheels by hand to hear/feel where the noise is comming from or if there is any drag from pads not returning back (touching rotor while in motion).
Are the pads OEM installed at the dealer and did you had the rotors cleaned up (cut)?

Thanks for your reply mbenz80. No, I have never done any off road driving with the car. Also, the pads were not fitted at the dealers, but they were original pads bought from the dealers and fitted by a garage (kwik fit). Strangely, the noise only started three months after the pads went on. As suggested, I'll take the wheels off, it's just knowing exactly what to look out for (a mate of mine suggested applying some grease to the wheel bearings).

There is no way to apply grease to wheel bearings on ML unless you remove the bearing out and take it apart because it's sealed from both sides. It took me 4 hours to replace one (right rear), it was still the original that lasted 157,000 miles but I use my ML to haul cars and pull snow plow so I was not surprised.
I was talking to mechanic hwo used to work at Mercedes dealership now working only Porsches and he said it was very common repair on ML trucks invoicing 6 work hours per wheel bearing for labor + parts.
I have 3 more new OEM bearing ready in case another one starts to make noise which on my truck sounded like a helicopter or motorcycle behind you. I've installed new set of rear brakes at that time (about 1,000 miles ago) including parking brake kit and so far all very quiet!

I just had my 2001 ML320 (164,000 Km) repaired for what I heard as rear end noise.
It was the center drive shaft bearing. I also need a RR upper balljoint and Tie Rod.
Since I had to be towed ($355,00) Check that. By the way, the bearing had been replaced at 100,000 Km.

could be E brake shoes rubbing, also heat shield noise is commmon on your ML

Check your e-brake pads. One of my springs came loose and the pads and other parts were grinding inside the rotor.