The trolley is responsible for pushing or pulling the opener arm along the chain so as to open and close the garage door.
Garage door chain tight one side loose other chamberlain.
The one you re looking for is attached to a bolt found at the spot where the trolley or carriage connects to the rail.
Do not over tighten the chain.
Step 2 turning off the power.
This will cause premature wear on the chain rollers.
Once you are clear that the only problem is a slightly loose chain then you can begin fixing the problem.
Adjust it tight enough so that the center of the chain sags about 1 2 down from parallel with the track graphic below right.
Make sure that the garage door is closed and that you have another way of getting out of the garage.
If in doubt replace the chain completely.