I recently had a Mr. Slim installed. My question pertains to the indoor unit. There is a high pitch background sound in addition to the airflow sound coming out of the indoor unit. The high pitch sound is subtle but still easily noticeable and, frankly, and little annoying. The pitch fluctuates with the fan speed; thus I assume that it stems from either the fan motor or the intake air coming into the unit. Is this a normal operating noise? Can anything be done to eliminate or reduce this background sound when the unit is running? Any suggestions would be greatly appreciated.