M304D Odd noise just after unlocking

[sofamanuk]

M340D MY21.... it has started making an odd noise a few seconds after unlocking (approx after folding wing mirrors are done). Not the same as the other random noises under the bonnet - seems to be from the rear...

https://youtu.be/jXATx1jPCG8

If you then lock and unlock then it doesn't happen, but leave it for 3mins or so then it will do it again.

Any clues? Will I spontaneously breakdown at some point?!

[sofamanuk]

(Sorry about the typo in the subject :-)