Machine gun sound on radios

[patt1345]

I work for a fire department and recently took over some radio maintenance responsibilities. We are currently having a issue with most of our station radios (XPR 4550 and unication G5 pagers). The issue is that at night our stations switch the station radios to “night call” which is basically a pager channel that only activates the radio when that station is toned out. Well over the past few months we have noticed a loud machine gun sound after the station tones have been activated. Voice transmissions still come through clear, but when no transmission is being made the machine gun sound is pretty constant. We operate off of UHF analog for our main channel, but do have 2 digital channels and a DMR repeater. I have heard some theories, but was wondering if anyone had a good solution. Thank you!