Monitor cables have a big lump adjacent to the video
controller connector. It is a ferrite filter. You could try
same. Filter would be wrapped by power cord adjacent to where
power cord attaches to printer. Another test would use
braided shield that encases the power cord and attaches (is
grounded) to printer chassis (not to be confused with
printer's electronic ground). These tests to determine source
of noise being from printer electronics.

I hope this is consistent which what you have posted since
the sentences (especially in last paragraph) are not specific
as to which cable is and is not connected when noise is and is
not experienced. IOW if USB cable is or is not connected,
then noise levels do not change?

Warning about measuring noise levels. Sound (amplitude)
from radio does not accurately indicate amount of noise.
Better measurement is to determine distance from 'noise
generator' to get diminished noise. Longer distance means
transmitted noise is greater.

tommyknocker wrote:
I tried unplugging the computer and turning off the power supply. No
luck. But unplugging the printer worked. My printer is a very low
end HP that has a giant power supply attached to the plug and a wire
leading from the PS to the printer. Note that the problem is NOT the
USB cable connecting the printer to the computer but the printer's
power supply. The power supply to the PRINTER, not the computer.