The Trek (and all OEM mid-drives AFAIK) has a geared motor (look at the ebikesca tutorials for an explanation), and they're noisier than direct drive (DD) motors (mostly confined to the DIY crowd, Stromer and other hub-drive systems). IMO, the quietest motor will be a DD with a sine wave controller. A Brose mid-drive could be the quietest of the bunch since it has steel gears and can use a belt drive instead of chain for reduction.
Last edited by 2old; 08-09-19 at 12:02 PM.