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It could be line-conducted interference as well as RF.
To find it, you need to turn things off and see if they are the source. Anything with a switch-mode power supply - your computer or whatever - is suspect. Include LED and fluorescent light bulbs.
If you do find the culprit, plugging it into a different outlet (line-conducted) or just moving the set away from it (RF) could help.
45.75 MHz is the standard IF for all analog sets since the late 40s when 20 MHz IF designs were abandoned. All brands of NTSC color TV use the same IF frequency. This is because the FCC analog channel assignments were predicated on avoiding image frequency interference assuming sets had that precise IF frequency. The spacing of 6 channels between UHF stations in a given city depended on it.
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