It is a Uniden BC92XLT.It scans all the programmed frequencies but will sometimes stop on one of them and a loud GRRRRRRRRR like a motor and the sound changes pace then stops and then the Scanner starts to scan and broadcast all my Fire and Police Frequencies.
My guess it sounds like you may have some intermod coming from somewhere, or...
Whatever frequency the noise is coming from is mixing with your Fire and Police Frequencies via the Intermediate Frequency in your scanner.
Check your owner's manual for the intermediate frequency. There is some sort of math you have to do to confirm if the noise on the offending frequency and mixing with you frequencies you wish to monitor in your scanner. The trick is finding the offending frequency. Having 2 scanners helps in this matter.
Try searching in the 450-470 band first for the noise.