My fan made a high-pitched whine which got worse over time. The people I talked to said that a lot of times, the heaters are noisy because of hornets building nests inside the units. They are attracted by the warmth. (You can buy a screen that fits over the heater vents on the outside of the trailer and this is strongly recommended in most areas.)
So I took a look at taking my heater apart. There was an exploded drawing of the heater in the installation manual that came in my packet of Casita
stuff and it sure didn't look easy. And then there were the safety issues of putting it back together wrong. So I had a professional work on it. In my case, the fan motor was worn out and I had it replaced. It wasn't cheap (about $250) but I sleep much better now.