the system you are scanning is an analog "simulcast" system meaning that your receiver is likely hearing more than one transmitter site at the same time. Hard to describe, but the simulcast can produce a slight 'swooshing' sound in the background sometimes.
Another cause of interference could be a nearby cell phone tower. Although the frequencies are somewhat separated, if you are really close to a tower, it can interfere with the signal. Might want to try changing the location or orientation of the antenna, assuming you are using the stock retractable model that comes with the scanner.