Hi,
On the return flight everything was normal (door had been opened and closed during turnaround) so I conclude it might have been some kind of debris in the seal. Strange thing was that it started shortly before descent in the outbound flight, which was at about after 30 minutes in cruise level. We informed engineers and next crew at home base, so they would be aware for the next flight. Of course we had perfectly normal cabin pressure and v/s indications all the time without any kind of vibrations.
Thank you for sharing your experience.