Is it coming in on the antenna? Disconnect antenna to find out. Is it coming on power (ac) connection? Run on battery to find out, or on a known quiet inverter fed from a battery.
If you have a handheld that can listen on MF and HF, walk around with it a bit. Most, or at least many, modern VHF and UHF handhelds have wide-range receivers.
Stuff like that, which probably you already tried but didn't mention recently.
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-- Carl
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