Most of the time this is a 'loop-area' problem.
Try this:
bundle
all the wires together -
AC cables & audio cables so that there is
no loop. use nylon ties. Just
one big cable. Where the wires go into the monitors, tie the audio line to the AC cord so that it is only one 'cable' going to each monitor from the converter or monitor control box.
No stray cables or bundles are allowed.
See if the noise is reduced or eliminated. - Let me know!
Cheers,
John