If I remember correctly, an 18 tooth is the equivalent of (roughly) a 29-tooth rear sprocket in first gear. (about 60% of direct drive, or roughly 1.6xsprocket size)
As such, you probably aren't thrashing the hub enough to have damaged it too badly, and even if you had, I'd expect the issue to be at its most severe when you did stand, which you've said it isn't.
What crankset are you running? Possibly more importantly, what chainring? When I had a singlespeed setup with a chainring designed for use with a front derailleur, it began making a noticeable noise on every rotation of the cranks. I suspect it was the slightly different profiling of some of the teeth (they do it to improve shifting, I assume) that was causing the sound. It went away when I got a proper singlespeed chainring.
Originally Posted by
fietsbob
maybe an inherent sound of the pawls in one of the gears not engaged
clicking.
AW3 speeds do that in high, the low gear pawls click as they pass
The S3X doesn't have any pawls, if it did it would have to freewheel in at least one of the gears.
At least I assume it doesn't, the ASC certainly doesn't.