hello, everybody

My wife (who, by the way, is the primary driver of our S90) started complaining about the humming sound she hears when idling at traffic lights. If I had to describe it, I would say that the sound is kind of similar to the PAIR pump noise which can be heard during the first 70 seconds after the cold start. But I don't know yet if it is caused by the PAIR. This is a low humming noise. It comes and goes (not constant). Most of the time I can hear it comes during idling at an intersection (especially if the radio is turned off :) It does not happen at every intetsection. Could it be the PAIR pump? Is this normal? I've read that the PAIR system is activated mainly when the engine is cold. I figured, the pump should not be working at normal operating temperatures. Anybody else can hear the low "humming" noise?

Raphael