Sorry for the trouble you are experiencing. Hard to believe 3 units in a row all exhibit the same behavior. Mine is no louder that what I'd expect for a laser printer. Don't have any suggestions as you've already replaced the unit and toners many times.
~Rick
Bay Area - CA
~R5 C (1.1.2.1), ~R50v (1.1.1) ~RF Trinity, ~RF 100 Macro, ~RF 100~400, ~RF 100~500, ~RF 200-800 +RF 1.4x TC, BG-R10, 430EX III-RT ~DxO PhotoLab Elite ~DaVinci Resolve Studio ~ImageClass MF644Cdw/MF656Cdw ~Pixel 10 ~CarePaks Are Worth It
