Radio and heater/Ac hum 05 prius hybrid

[Kblue]

My heater/AC makes a high pitched hum when my radio is turned on. Turn off the radio and keep heater A/C running, the hum goes away. Or, turn off the heater A/C and keep the radio playing and the hum will also go away. Use both at the same time....high pitched hum. Any ideas to fix this or what causes it?