It's a chain. you should be looking to replace it soon!
When you remove the cover and see how much slap is in the chain at this point you will be glad you did......you must remember this is an american car not a crappy import, the engine will not like a broken or jumped tooth if it wears out, like an import with a belt they have tolerences that might allow for a broken belt to allow you an engine that just needs to be retimed and have a belt put on, but your awesome mustang has a real valvetrain and as the chain stretches your timing gets outta whack a bit, not a big deal but at the same time at this point since you would have the water pump off too, id change that too!
advice from many many years of ford experience!
Thanks and hope this helps!