My new 900U emits a 'grinding' noise from the filter side of the unit (opposite of the fan side). It isn't constant, but it does occur at startup, and periodically thereafter. It happens in normal fan mode, but not in high fan that I've noticed thus far. It is very distracting.
Is this the infamous auto iris noise that I've read about? Seems like if it were the fan, it would be located closer to the main fan, or are there multiple fans in the unit?