Steady Buzzer Sound

I have a 1994 Chaprarral 2130 Sport that I bought new and have kept it in my garage ever since I took delivery. I only boat in a freshwater lake, wash and clean it up after every trip, so this issue should not have anything to do with corrosion.

The boat came with the 7.4L Mercruiser which threw a rod at the end of 2020 with almost 1,200 hours on the engine. A friend recommended a guy who builds high performance engines for boats and race cars. I looked into that and for just a bit more money than doing a complete rebuild, as of March 2021 I now have a cusotm-built 496 stoker engine. Everything ran great all last summer, but at the end of the season, an electrical issue started happening that I can't figure out. Coming home from the lake, I stopped to top off the tank and heard the buzzer. This is the one where if you turn the key on and don't crank the engine within about 15 seconds, it goes off. The only way to stop it was disconnecting the positive cables from the battery.

Over the winter, I was passing by a West Marine store and picked up a new ignition switch. Yeah, I know that when there's a problem, you don't just start swapping parts but the switch was only around 20 bucks and seemed a good place to start. Put the new switch in and yep, same problem. I have traced all the wires from the ingition swtich and don't see any shorts or bare wires rubbing up against anything sharp.

Any ideas what to check next? Could this be the starter solenoid and how would I check that?

If its the audio warning horn, then it sounds like the wiring in the dash has gotten loose (maybe).

reconnect battery and then find the horn, if its going off - if its standard mercruiser stuff, it (the horn) should have a purple wire (likely from one of the instruments and a tan/blue wire with a bullet connector. separate the bullet connector and hopefully, the horn should shut off. take a voltmeter and probe between the purple wire and a black wire at the instrument where the purple wire from the horn connects. If you have +12VDC, then something is providing current to the instruments (should be the key switch but that should still be OFF).

If you have another set of eyes, they can watch the instruments when the battery gets reconnected...

Thanks for the reply but one thing I forgot to mention (sorry!). When I reconnect the fat red wire coming from the starter back to the battery, the console lights and gauges come on. About 15 seconds after that is when the buzzers sounds. This would be the exact behavior if I were to just turn the ignition key on and not start the engine.

ok...sounds like there is a short between the battery's positive loop and the ignition circuit...

I would start by separating the large connector in the main engine wiring harness (connecting the engine to the dash)...that will give you some indication on which 'side' the issue is..

Forgot to update this thread after finally figuring out the issue.
There is a clip on the side of the engine block that holds up the wiring loom behind the alternator. The clip was loose enough to allow the loom to sag down, which caused the orange alternator wire to just barely rub up against one of the studs on the back of the alternator. The wire was just barely punctured by that stud, but it was enough to cause the short.
I appreciate all the hints and tips!