probably a flow or temperature sensor sounding off... at idle there is very little flow. if the impeller (either engines circulation pump or the raw water pump) is getting tired, it could easily happen at idle, and that is most often when/where you'd get your first warning.
Comments
Dream 'Inn III -- 2008 400 Express
2002 FV 342 on Lake St. Clair - Past Commodore SHC - Vessel Examiner USCGAUX