We have a 2007 Pilot 34 with twin Yanmar 240 engines, and have had a similar event happen. It started barely audible, then would get louder over time. All gauges were fine, manual checks were fine also. The alarm switches on the Yanmar are separate from the sending unit for the gauges. It turned out to be wiring to one of the alarm switches shorting to ground.
It was very frustrating trying to figure out what was going on. We have an audible alarm for each engine that covers temp, oil pressure and gear temp. From the helm you couldn't tell which engine was alarming, or for what reason. I ended up eliminating the problem by converting the gauges to NMEA 2000 and using Maretron display to monitor everything.
Assuming you have a Yanmar, the switches go to ground when activated and are connected together so any one of them activating sounds the alarm. This makes troubleshooting more difficult. Search for a thread from RGano on his solution, or if you are interested in the NMEA 200 approach, I have a thread on that.