Isolation is the problem. Grounding and shielding are not right.

Hire a competent electrical technician to sort it out.

Thanks for the reply Lyn,

All grounding is good, rope lights are a bit difficult to shield though as you won't be able to see the lights if you do.

Hi John,

Thank you for taking the time to respond, I will give it a bash and see if it works.

True, dat.

If practical, the PLC program could be temporarily modded to drive one circuit only (the longest) - then have a listen to the results downstairs. If it's still noisy (as expected), see if the contact suppression fixes it.

Thanks PWSlack,

You guys have been very helpfull, much appreciated.

Thanks JRaef for the info,

Much appreciated.

I agree with what has already been said but there is also the possibility that the sound system itself may have a hidden problem that this signage has revealed. Try turning ON these panels one at a time without using the PLC system. The sound system may have some floating inputs picking up noise, bad shielding or simply a poor power supply design (susceptible to line fluctuations) that was not noticed earlier.

I would also ask If I could have the sound system checked as well....

This may prevent future problems from cropping up if they have have poor shielding grounds, el-cheapo amp, etc...

The interference in the PLC panel is alow frequency(less than 100Hz) as you said like a fan-effect so it isn't transmitted to a distance like some 10 meters or long .the channel that is more effect and almost makes passing through is the wire of the power electric .so put a filter to less the effect between the source and the PLC power A.C input .to prevent the effect back to the source.

Also try putting an isolation transformer between the power source and the sound equipment and make sure the sound equipment is grounded well. Separate the sign power from the sound system power. Also, make sure the sound system audio cables have shield ground connected ONLY at the amp end of the cable. This eliminates ground loops in the audio transmission cables.

If any of the audio equipment is powered by a lump or a brick, then loop the low voltage power line from the lump/brick through a ferrite core 2 or 3 turns. This will suppress high-frequency signals in the supply from reaching the equipment.

Try a AM/FM radio, or a guitar amp. (without any input or control cables attached), etc., powered from the same outlet as the Audio System to hear if the "noise" is coming through the power source. If OK, then power up the music system amp. with all inputs disconnected, turn up the volume a bit, listen. If ok, turn down volume and install one input at a time, repeat process. This may help locate the offending source and expedite the cure.

Thanks Guys, your help has been much appreciated here.