I have a similar issue on 857.9625 MHz. It happens everywhere on my Pro-96. I do not have this problem at all on my Uniden 396T. And no it is not interference from a nearby computer, cell phone, etc. This frequency happens to fall in the SAFE-T system and has made my Pro-96 unuseable for montioring SAFE-T in District #52 in Indianapolis.
FWIW, my Pro-96 did not do this until I had it for about a year. I probably need to send it in to be checked out. If it really is a birdie, I'm pretty much SOL I guess. It's too bad because otherwise the Pro-96 is a great scanner.
GTO_04