I have a late E60 with the factory fit electric tow bar and have had it replaced by BMW after rear collison. The new tow bar was "chattering" with the Al-Ko 3004 stabiliser on my caravan so at first I thought it was the friction pads. Turns out the 50mm tow ball is actually 49.81mm and Westfalia have failed in their quality control I now have a aggrevation of convincing dealer that its a fault and needs replacing FOC.

 

Good luck. You realize that you're pissing on about a 0.095mm difference on either side of the tow ball? I used my 50mm Westfalia tow ball to tow a US trailer that normally uses a 2" (50.8mm) tow ball. That's 0.4mm of side slop and my 4000 km drive was no problem at all.

The 'chattering' you are hearing is the tongue weight being too light and thus allows the trailer coupling to bounce on the tow ball. Put more weight on the tongue. This will force it to stay down and planted.