- The Fan Blades Are Bent - sometimes foreign objects can get into the air conditioning using causing the fan blades to bend. Check to make sure they're all straight - if they're not, you can straighten them until they are properly aligned.
- There's Something Inside - no, we're not talking about a creature of some sort, but we are talking about foreign objects that could get into the system. After all, the majority of the unit hangs out the window so there's a good chance things like sticks, rocks or leaves can find their way inside. You can look for and remove these objects by opening up the front grill.
- Loose Grill - hearing excessive vibrations or rattling? It could be coming from the front grill if it's not sitting securely. Most of the time, they're held in by a simple locking mechanism so if the grill isn't put back properly, or if it's not locked into place it could cause noises to occur.