High Pitched Electronic Noise from AC/Heater Unit

I started to hunt for help on the Greater Internets but realised that this was the sort of thing more suited to this crowd.

In our apartment there are electronic combo heater/ac units. One of them emits the classic, high-pitched 'electronic" sound so i turn it off all the time which annoys my spouse whose hearing isn't as acute.

Has anyone advice about fixing the appliance before it breaks my marriage?

I can go into more detail as far as set-up of the rooms if needed - make of appliance, location of other electonics...

Thank you so much!