Bluz, It sounds as the system is notcausing the noise but amplifing an induced noise.
Did you install the station cabling?
It's possible that some of it may be ran parallel
or strapped to some AC conduits or maybe laying on top of florescent lights.
also have you isolated the power to the unit with a good UPS?
Is it possible to go without those stations for a few days?
It's still possible that as JJimm had mentioned that a chip or some other IC is heating up and with those 3 disconnected the load isn't great enough to heat it up.
You may want to swap those 3 stations to another card. If it's 1 or all of those 3 then it should follow. If it's in the IC then it will stay at that card regardless of the field wiring as long as it has the same load on it.
Just a though.
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MrG