Buzzing alarm on 70hp

I have an 1989 70hp that will start a solid buzzing sound (from the console) when at idle or *****ing after about 2 minutes. The engine then chugs (while alarm still sounding) when throttled up for another 30 seconds or so until the alarm stops, which is when the engine immediately begins running smoothly. As soon as the engine returns to slower rpm, the alarm comes back on and chugging begins again.

The water pump seems to be working as the flow is solid and strong.

Any help is appreciated.
Thanks!

[boats.net]

The warning will activate if oil level is low or if engine overheats, probably overheating.