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It may be necessary to run balanced audio out to the TX if other methods to not remove the problem.
Radio Shack has a few inductor core forms and you can make chokes by wrapping a few turns of that audio cable around the inductor core. Best place is right at the transmitter just before the audio plugs into the transmitter, and if necessary, again at the beginning of the run of wire downstairs.
You may also have to do the choke approach with the power cable as well.