"randalln123"....In 2005, we bought a brand Monaco Diplomat from a dealer about 150 miles away. On the way home, our first drive, we had the same sound and it increased until it became obvious that it was an alarm under the dash.
We stopped the coach and called Monaco. They told us it was an engine temp alarm installed by Monaco aa an auxiliary to the Cummins alarm. We were told to disconnect it, and we did. It was never an issue and all of our gauges worked properly for the 10 years we owned the coach.
It starts out faint and is scratchy sounding, slowly increasing in volume. It's located under that removable dash panel covering your instrument cluster. You'll have to hunt through the wiring looms to find it. It's been 15 years since it occurred, so I don't recall what it looked like.