Our 1988 W124 wagon is making an intermittent humming/droning noise ONLY when moving in the vertical plane.

That is, while driving over dips and mounds, the humming/droning occurs, but
immediately goes away.

It does NOT do this during turns or under acceleration.

A check of the front wheel bearings and pulling & rotating all wheels including shaking them hasn't shown anything.

Any ideas ?

Thanks,
:-) neil

1988 E36T AMG
1993 500
1995 E320T