[RFI] MFJ-856 oddity

[N1BUG]

I'm still pursuing my power line noise. I have a few questions, 
starting with this one:

A while ago I modified an MFJ-856 power line noise sniffer. The mods 
consisted of adding a T match to the driven element, moving the 
receiver back behind the reflector, and adding a 3 step attenuator 
(25, 15, 10 dB).

Most of the time this behaves as expected, but every once in a while 
I find a pole where the attenuators do nothing whatsoever. These are 
not always situations where the noise is so strong that the meter is 
full scale, but sometimes that is the case. At first I was paranoid 
about bad switches on the attenuator, but this pattern is repeatable 
at certain poles and happens at less than 5% of noisy poles. When 
this happens the sound is more like a hum than other poles. By that 
I mean less like a buzz, frying or other variations. So what is 
going on? The only two theories I have - and I fail to really fathom 
how either could be happening - is that 1) Noise is coming down the 
coax through the attenuator and into the receiver common mode in 
rather than differential mode or 2) a strong induction field 
coupling to something in the receiver or headphones. Sounds crazy? 
Any thoughts?

73,
Paul N1BUG