I cleared the air, (love that about Iwaki's...they clear the system so fast
), and opened the pump, it looked very clean inside, no problems that I could see, the ceramic bearing looked fine also, and the O-ring was seated properly.
The thing is, it's been running for almost a year like this, no worsening or lessening of the noise. The only time it is dead silent (just that turbine like whirr) is when I run it in a bucket with no backpressure at all.