You should keep the topic in one thread so all info is one place, folks don't have to ask about things again and again.
The horn needs dc from a bat., a whistle relay only closes points between batt, and horn, like a switch.
So is the relay chattering on/off/on or is the horn not getting good battery power thru the points? Clean and sand/file between the points where the touch when closed.... lightly, straight, flat, level... they should be flat&smooth and touch level when closed. Little tits can build up on points over time as jumping electrons(etc) drag a few metal molecules across the air gap(s).