Vibration in reverse

Mines had something similar to what you're describing since new. Audi took it in and as far as I can tell have greased them up as blue grease appeared on the brakes on the cars return.

The noise usually only shows itself when in reverse after it's rained the night before but not all the time. I thought I'd hit something the first few times I heard the noise as it is quite loud. I can hear it when I gently tap the brakes at slow speeds reversing. Never heard it going forwards.