Pulled unit out of wall to pinpoint source of noise. Found a broken propeller type fan. The ring that connects the three blade tips was broken into three pieces. The fan was way out of balance and was causing all the racket. For a temporary fix I cut away all remaining pieces of the ring so the fan was back in balance. Ordered new propeller fan from AppliancePartsPros and installed. The white plastic shroud must be removed to get the fan assembly out, about a dozen screws. Took about an hour altogether. Sounds like new.