If in doubt replace the chain completely.
Garage door chain loose when closed.
A loose garage door chain has less power to open and close the garage door and it can often create a loud and annoying sound as the slack hits the door.
Make sure that the garage door is closed and that you have another way of getting out of the garage.
Once you are clear that the only problem is a slightly loose chain then you can begin fixing the problem.
Step 2 turning off the power.
Tightening up the slack on your garage door will allow it to run more efficiently and will prevent loud noises that disrupt your home.
Over time though this slack can increase or the gear and sprocket assembly can wear and the chain may pop off the sprockets.
Chain driven garage door openers should have some slack in the chain for optimal performance.