I wouldn't expect an FM filter to provide any relief for interference issues up in the UHF band, the math makes that fairly unlikely. Similarly, I also wouldn't expect 600 MHz cellular to cause interference down into channel 23. However, I've been surprised before by things that I hadn't anticipated....
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... or the sun going down is helping.
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If it returns when the sun comes back up, look for a solar-powered anything. If the interference abates during cloudy weather, that might also suggest a solar powered interference source.
Rabbit73's suggestion regarding the AM radio might help, but if the noise is restricted to only one UHF channel, it may not be sufficiently broadband to register down in the AM band. It is worth a try, though.