I run two Airseps together too and there is a hum, but it isn't very loud...like Chad said you can have a normal conversation. I really don't even notice mine anymore.
I got one of mine from Paulette at suncoast beads, but she has turned the reconditioned portion of the business over to Margi at
oxyunlimited and I got my other one from someone here at LE in the garage sale section.