On my own ship, over the years I have noticed a few revs... and they are few... where the motor tends to be smoother. At some revs, the ship vibrates and the compass pedestal in particular notices it. I have learned to live with it. It's hard to tell the degree of vibration you complain about.
Take your motor revs up until it is at its worst, then send a man below to see what might be doing it. You might see something then.