What kind of interference or other receiver issues can cause a constant buzzing noise (picket fence line in the audio spectrum) in the demodulated audio of a received NFM VHF signal?

With AM, it could be additive RF noise, but captured FM demodulation should ignore that?

This particular noise is not due to a transmitter issue or PL tone or weak FM capture, as the buzzing appears and disappears depending on the location of the receiving station, not the particular transmitting station or receiver equipment (2M HT or SDR) used.