Pro-652 background "growl" on NFM mode

[wbswetnam]

My Pro-652 sometimes has a bizarre background noise, kind of a "growling" sound, on distant stations in NFM mode. What causes this, and is there a fix for it? I've never encountered this on any scanner I've ever owned... and I've owned a heck of a lot of scanners over the past 35 years.

 

[Nasby]

You could try switching them from NFM to FM.

I've tried both settings on NFM channels and can't tell any difference at all.

 

[wbswetnam]

It only happens in the VHF-High band (i.e., 150 to 159 MHz). It never happens on UHF frequencies. Could it be intermod with some other signal? if so, then why doesn't it happen on other scanners?