Hi mideakins,
Welcome to the Heartland Owners Forum and Family. We have a great bunch of people here with lots of information and all willing to share their knowledge if needed.
I would get your dealer involved in this as soon as possible. Probably not anything loose in the ceiling or roof, but maybe something in the A/C, like dobber nest, or something like that. The sound will travel and fool you, but don't take any chances, call your dealer and have them check it out.
Enjoy the forum and your new BigHorn.
Jim M