I have a 2008 Challenger 230 with twin 155hp engines and this past summer I would also get the LOUD beep after starting the second engine. It did not matter which engine was started first, the annoying beep would happen almost every time I started the second engine. My LOUD beep was resolved when I charged the battery until it was fully charged, then no beep. But if the boat sat for a week or so, the beep would come back which confirmed it was a low voltage warning when starting. I installed a new battery in August and that helped cure the LOUD BEEP after starting the second engine. For next season I will have solar charger at the dock to keep the battery fully charged. I have a CanDooPro and it is great for diagnosing SeaDoo issues, writing keys, resetting maintenance reminders, etc, I highly recommend getting one if you are keeping the boat