Humming from Phono Input on McIntosh C33 Preamp

Does anyone know why I would be getting humming / buzzing sound from my speakers when I toggle past an "open" phono RCA jack port (no cable in RCA jack port) on "listen" device selector switch on my McIntosh C33 preamplifier? No hum off of any of the other device selections, which are are being used by tuner, turntable, CD players, etc. Sould I use shorting plugs in the the un-used L & R RCA jack ports for 2nd phono to stop this from occurring?

Thanks for your help & consideration.