I know absolutely nothing about garage doors so I appreciate the help. I recently purchased a new house with a 2-car garage. Instead of one large garage door, each side has a separate door. Both doors function properly. However, I recently noticed that one door made a twangy, flapping sound right after it closed. Upon further inspection, I noticed that the belt that runs along the top was sagging down. The other garage door's belt doesn't sag at all and seems to be in a track. Again, the door operates properly but I'm wondering if the sagging is something I should be concerned about. I added a picture to show the sagging the belt. Again, any help is appreciated.

Your drive chain is in need of a adjustment. There is a adjuster on the chain with which you can easily take up the slack on the chain. You will need a small adjustable wrench or a pair of pliers. RC