Whatever you are hearing wasn't evident on a PC here....I listened to your feed for about an hour (just out of curiosity) PD traffic sounded fine. Only humm I heard was from one of the dispatch positions, sounded like a fan running in the background, but it was low.
I just moved to a desktop PC with a good set of speakers.. I can hear it - I believe it's their CTSS tone (146.2) that you hear. Need to roll off the bass in your audio coming from the scanner? It does not sound like 60 hum or 120 cycle buzz from the AC, the pitch is higher.
Edit - It is their CTSS tone - I ran this tone generator
at the same time with the level set at the same as your audio. D below middle C is 146.83 - when both were on at the same time you could hear the slow beat frequency.