My father just bought himself a CLS 320 cdi, and I am probably taking over his E270cdi (2003).

This is now making a deep sound sometimes when driving in a stable speed. It happens when driving slower than appr 60-70 km/h (40 miles/ hour ?)

It happens wheter or not I use the cruise control. It happens mostly when the engine is not working hard, and when there is little pressure on the gears.

The sound is like deep tone, like a deep humming.

The engine also starts ajusting rapidly up and down the revs. May be 100 revs/minute up and down. Like from 1700 to 1600 and back up and down.

A person I asked told me it could be a "electro thing" inside the automat, and/ or some flow measure instruments inside too.

The workshop has flushed the automat twice. It got better for a short while, but is not good anymore.

Any ideas or experience with this?