The shrouds (wires that keep the mast from falling sideways) are covered with a black tube on most catamarans. It is there to prevent chafe of the mainsail when sailing down wind.
The problem is that this tube can rattle in the wind. Not all tubes do that, only the ones that appear straight. It is very noisy and carries through the whole boat. We only had the problem on the port side. Initially I solved this by wrapping the spinnaker halyard 5 times around the shroud but that was just a temporary fix. To solve the problem permanently, I drilled 6mm holes every 50 cm and filled the holes with black sika. Best is to do it on the side of the shroud that appears furthest from the wire. Do it on a day without wind so that the Sika has time to harden. Don’t forget to take a picture of your boat when you are up the mast.