Chainless doesn't mean that it still isn't your BB. Pedaling with a chain the BB has to react to offset forces from your weight left to right in addition to chain tension. That force vector changes directions as you pedal and the creak can be caused by that, Chainless the BB reacts only to your weight and for the most part laterally balanced.1) 40 x 14t is kind of annoying to be in all the time. There's a woman with a custom road frame with a single speed belt drive setup who works on the same campus as me (maybe at Children's Hospital? not sure), and she seemed to have a much shorter gear than I'd expect and just made up with it in cadence.
90 rpm works out to 21.3 mph for 40 x 14t, 19.9 mph for 15t, 18.6 mph for 16t, 17.6 mph for 17t. I should probably go with a 40 x 15t equivalent, or maybe splitting the difference between 15 and 16t if that's possible with the belt. Edit: 40/16 is 2.5, and I could get 2.5 with 55/22, 118 tooth belt, 433 mm chainstay.
2) Also related to this belt drive business is that my current 1x10 drivetrain is driving me nuts. I have a clicking type noise with every crank rotation. Same place in the rotation. Doesn't occur with no chain so not the BB, crank-BB interface, or pedals. As far as I can tell I have simply worn down my Race Face narrow-wide ring. I bought it 11 months ago and have about 1200 miles on the commuter since then per Strava, so perhaps that's its lifespan in these conditions.