I cleaned up the sound:
http://test2.ergonomie-advies.nl/images/Noise2.mp3

Can it be a bad wheel bearing when:

1. It only appears after 1 to 5mls (depending on the outside temperature).
2. Is not affected by cornering.
3. Does not get much louder at higher speeds.
4. Gets louder if I ease off the gas
5. Seems to disappear (at least gets quieter) if I accelerate.

Rob

Sounds like my speakers are under water. :) Noises can be difficult to locate.

I wonder it a piece of metal / heat shield / catalytic converter is resonating. Have you driven by someone and had them listen for the noise?

I even had 3 different (very experienced) mechanics in my car to listed, so far without result.

It only makes this noise when driving that makes it harder to listen.
With microphones under the floor insulation, I found that the noise is the loudest near the accelerator pedal.

It comes from the underside, it is also gets louder on a rough road.
Noise sample on rough road.

I used a rubber mallet to tap everything on the underside to check for resonances.
I even had the car on a vibrating shock absorber tester.
No suspicious vibrations found.

Rob

My bets are wheel bearing. I had a similar noise on my GLK not too long ago and it was a front wheel bearing. Could also be the 4matic axleshaft (boots in good shape?), but I bet its the wheel bearing.

The boots look ok.

If the axleshaft is worn it would probably make noise if I let the car run with the wheels off the ground, but I only hear it when driving.

Rob

I replaced the wheel bearing, the old one had a bit of play.
The races looked smooth.

The noise has changed but it is still there.
It sounds now more like excessive tire noise or suspension vibration, especially on a rough road surface.

The next step will be replacing the hydro bushings.

Rob

Try rotating your tires and see if it changes.

I already tried that, made no difference.
Changing from summer to winter tyres also made no difference.

Rob

brake torque the engine and see if the engine is lifting excessively on the driver side.