Question: what the heck is this electric motor sound when I unlock the backrest ? I suppose it should move something, but I cannot see any move

 

If you mean when you tilt you seat forward to get access to your rear seats then the sound is your headrest retraction

 

Or alternatively, your headrest is not retracting, but the motor spinning is anyway because the motor and the headrest have become decoupled. This is a common problem with seats (all six of mine) with a relatively straightforward fix. e.g. see this thread (and numerous others)

https://www.jaguarforums.com/forum/x...repair-225201/

and these videos

https://redirect.viglink.com/?format...%20-%20YouTube
https://redirect.viglink.com/?format...%20-%20YouTube

 

Yes, I meant when I tilt the seat forward. My headrests do not move, so I probably have this issue crbass mentioned. Thanks!

 

When I recovered my seats I removed the head rests, for two reasons. First my new seat covers did not have head rest covers and second I did not want to put holes in them.

As a result I get that motor sound every time I unlock the seat.