you need to put a FM Trap / notch filter in your coax, just search google or even eBay, there are a lot of them, Good Luck.
put it between the antenna and the scanner closer to the scanner is probably best.
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When electronic components are exposed to signal levels they are not designed for they will go into overload and in a non-liniar mode that makes them work as mixers. Two signals mix and creates a third signal, or several, that only exists in that component. It could be a receiver that are badly designed or use inferior components or the signal strenght are far too high.
Almost everybody will experiance an improved reception quality when using a FM trap filter.