You also have "snapping" when a channel/frequency activates as well. This sometimes is caused by a grounding issue with the power being fed, or "dirty power". You may need to look into obtaining a ground loop isolator. This may resolve both issues I am seeing on your feed.
Additionally, with the snapping I am hearing, this sounds like you have the scanner to have a two or three second hold time when a frequency/Talkgroup activates and concludes. Might want to consider reducing those times if you have a somewhat busy area being scanned.
Many times there is a compatibility issue between your audio source and the computer your using with it's audio card. What happens is that your audio source may have a DC bias voltage on it. When you connect it to the computer, this DC voltage can cause an issue inside the computer audio circuit. The simple solution is to just add a capacitor in series with the hot or high wire connection using about a 1 uf in size to block the DC voltage from causing an issue. The exact value is not that important.
But the value of the capacitor will effect the tone quality of the voice. Using too small of a value will cause more highs and limit the lows. Using a too large a capacitor will cause more lows and limit the highs in the voice.
Capacitors are cheap and easy to install. Let the group know how you make out.
Another reason that may cause hum is if you have the gain in the computer audio input cranked up too high. This can be corrected by having a higher audio level coming from your source. Then you can lower the computer audio card gain. Bottom line here is a balancing game to get it right.
Hum issues can drive you crazy. There have already been some good suggestions here but I will add a couple more things I have run into. These switching power supplies on computers, monitors, and accessories are notorious for leaking RF noise. Mix a dirty power supply with a little 60Hz magnetic field and it can put hum on audio circuits. Sometimes turning off and moving around power supplies can give you a clue if such a problem exists. Use shielded audio cables and keep them away from power supplies, monitors, cpus, etc. as much as possible. The other issue is any source of RF transmitter particularly on lower frequencies below 500MHz. Wireless microphones, speakers, baby monitors, etc...
Ground it, unground it, capacitive couple it, inductive couple it, shield it, move it around, and watch out for strong RF fields. Keep hacking away at it an you will eventually discover the cure.