My climate control has a high pitched cyclical whine but usually only when
the passenger side has air directed at the feet. When on defrost it's
normal, and on the driver side it doesn't whine if I select feet as well. I
originally though it was a blower motor but if there is only one blower
motor, and it's well behaved on defrost, what should I be looking at to
diagnose the high pitched whine?