A flat ceiling makes for a cleaner and easier crown-molding or cabinet installation. My method also is useful when blending an old ceiling to the ceiling of an addition. Stringlines guide the way. The first step in the process is to set up a laser level as a reference line (see Laser levels: fast and accurate). I think that you must have a problem with the balance of the fan.
![How to level a ceiling How to level a ceiling](https://www.doityourself.com/forum/attachments/basements-attics-crawl-spaces/59683d1449456266-finishing-attic-above-tray-recessed-ceiling-3.jpg)
That is because it weighted unevenly. Most new fans come with added weights to correct. When any framed wall is built the components or studs should all be hand sighted down the. Find out how to level a ceiling when remodeling an older home. Ceiling joists should all be all the same level along the bottom, but they can move over time.
If this has happened to yours, you should not drywall until you have added new joists to the sides of the old joists in order to level out that surface for the drywall. A basement ceiling in an older house will often not be level. The joists settle as people walk over them.
There is not a lot you can do about the problem except to level it by installing a dropped ceiling. My preferred method to level the ceiling is the same whether you use the traditional two-by lumber and strapping or metal studs. First you determine what the ceiling is doing, slanting this way or that, and find the high and low spots.