Hi Bekstam,
Welcome to the Heartland Owners Forum and to the family. We have a great bunch of folks here with lots of information and all willing to share their knowledge when needed.
You don't have listed as to what unit you have, but I am guessing most likely you are hearing the converter for the 12v system and charging the battery. I think most have a fan built in. And some units have a fan behind the fridge to help circulate the air behind the fridge and make cooling better and more efficient.
Be sure and check out our Heartland Owners Club. Join us at a rally when you can and meet lots of the great folks here and make friends for a lifetime.
Enjoy the forum.
Jim M