If the noise is only when the coach is in gear ; best guess is a "park brake on "switch failure .
The warning buzzer is trying to tell you to release the brake and they are , loud and annoying for a reason . It's a safety feature.
And the buzzer is hidden so it's hard to find and disconnect. So the safety warning can't be ignored.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.