Some of the 1900 guys (or former ones) can probably "chime" in on this
but if I recall the 1900 has some sort of interference problem when the props first start spinning. It leads to a very loud beeping sound that get faster until it eventually stops. Some times they would pull a circuit breaker that seemed to fix the problem. I don't know which one it was. But this all occurs on start up. So if this sound was in flight, ignore everything I just said.
Ethan