Are you sure it is the halyards and not the electrical wires? If the halyards are all tight, they should not slap inside.

Natural High was painfully loud until I re-wired the mast. What I did then, as every 3 feet I put 3 long tailed zip ties on the wire conduit with the tails in three outward directions. It makes for a lot of zip ties, but it holds the sire conduit in the centre of the mast and it makes not a peep now no matter how rolly the anchorage!