I've noticed that digital sound as well. It's always right after someone transmits and it stops as soon as the repeater PL kicks in (and then the repeater stops transmitting of course). Another agency could be operating a digital system on the input frequency. Recently I tried monitoring a few bus companies and was surprised to find they weren't transmitting analog or monitorable digital for that matter. Since the input frequency is 155.205, very common for transportation, maybe it's possible.