Dometic 15.000 A/C

My a/c has started to make a rattling noise after it has run for a long time. The a/c tech could find nothing loose or a reason for it to rattle. Of course after he left it started to rattle again. It only does it when it has been running for a while. I sounds like the frame is loose but that is not the case.....It sounds tinny but not the noise from high head pressure....