Urghh.......I have a 2 UM18 I bought last year and 1 right out of the box made a little noise when pushed to high excursions. I didn't send it in right away thinking once in the sealed enclosure it might not be noticeable. But it is. Anyway, the box is a dual opposed DIYSound group enclosure.
You mentioned the size change and that reminded me of others saying the new UM18s do not fit the flush fit/counter sunk cut out since they are bigger. Crap! What should have been a simple remove and replace the driver could be a lot of work and mess making the old hole bigger.
Maybe that is my sign to try ported
HTPC, Sony 40es, 120" Silver Ticket, 7702mkii, Sunfire Amp 225w, JBL 590, JBL 520
PSA XS30, Seaton Submersive, 2 Um-18 8cf sealed, Outlaw Ultra x 12, Kappa Pro 18LF, BFM Tuba 60 horn, B&C 18TBW100 6cf 41hz, 34hz, 28hz tune
iNuke 3000 & 6000 DSP's, Crowson Motion Actuator