[Jay-ZS+]

No simple fix, under the expansion bottle, a black box with the sounder unit in. Water gets in and makes the alarm go nuts. Split it open and dry it out then refit. Might be worth drilling a tiny drain hole in the bottom after you have dried it out and sealed it. The sealant is just for round the split line when you reassemble it.

Might want some earplugs when you go to disconnect it incase it goes off whilst your heads under the bonnet.