A very simple and cheap variable attenuator is available at radio shack.
They are connected via F connectors. Because your just receiving the F
connectors won't be important at vhf and below. If one isn't enough, I think
10 to 20db, purchase an in line fixed 20db to add to the adjustable.
I only recommend this because it's cheap and will get you what you want.
As for changing frequencies to attenuate the level, This works great.
However, in order to be efficient with this method, you must change antennas
as you change frequency. An antenna is designed to work or will work best at
a particular frequency. That frequency is where you should be tuned. The
effectiveness of the antenna is sacrificed when not on the resonant
frequency. The front to back ratio maybe effected to the point of causing
more of a problem than you want.
It's true that noises travel further at lower freqs and noises don't travel
as far when received at higher freqs. However, it is still important to have
a well tuned system.
Once your noise signal is received on your locator, increase the attenuation
to the point of barely hearing the RFI signal.
Best Wishes and Happy Holidays to all,
6469 Old Solomons Island RD
Tracy's Landing MD 20779