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Ramsey makes this processor for FM transmitters:
It is a kit so you would have to build it.
In the description they say:
Low cost CD players that have poor filtering on the output are really cleaned up by the switched-capacitor low pass filtering in the STC1. Higher order harmonics and clock feed-through that causes annoying heterodynes and whooshing noises disappear.
I don’t know if this is what will work for you or if you want to go to the trouble and expense to find out. You might do better on ebay if you can find a graphic equalizer.