Usage note: klaxon is a trademarked word that has become used generically, like Band-Aids, so you'll sometimes see it capitalized Klaxon.
My early-warning system set off an imaginary klaxon.
(The Dresden Files)
It was only on the third thought that she recognized them, as from the corridor outside, muffled by the door, an alarm klaxon began to hoot rhythmically.
Shards of Honor
Louis McMaster Bujold