I guess its residential on 1/3 of an acre lots. Not real tight.
But the utilities are above ground and that probably doesnt help.

not sure if anyone realizes this but i've noticed that the adj controls
can sometimes be very very sensitive sometimes i've found that a hair
kills the noise and if you blinked at that moment you missed it

dunno maybe it's just me

maybe you have a bad or dirty control? OR as Jan wrote previously the
variable resistor is not linear and the abrupt change you see it due to the
variable resistor.

the control is good, it is on all the units prob a non linear
pot(resistor) linear taper? not audio taper or what ever but they all
seem to work like that

on certain kinds of noise it sure works well

ml