Had a Fujitsu mini split system (36RMLQ) installed in my house last spring. Went with an 18k btu unit in my living room, a 9k btu unit in my bedroom and an additional 9k btu in the basement. Used it all of last summer and loved it! We've recently been doing a remodel project that required shutting down our radiant heat system, so we've been using the mini split in heat pump mode. (outside air temps in the 30-50 range) Been having issues with a buzzing noise from the air handlers. The 9k btu units seem to be the only ones effected and seems to get worse if the 18k btu unit is running. Now for the really strange part...the 9k btu units will periodicly buzz even when they are off and the 18k unit is on. Hvac tech said system seemed low on coolant so more was added, but didn't solve the problem. Ideas?