If that's what it is, don't worry about it, it's there on nearly EVERY mustang 6 or 8.
It doesn't affect anything. I've got 115,000 miles now, and have had no problems, though I'v ehad the ticking noise since day one. I had it on my 77 F-150, my 68 Mustang, my 82 Mustang, and my 68 Cougar.
It's nothing. you can spend a lot of money to fix it, but you won't gain anything related to longevity or performance.
Woe to him who builds his palace by unrighteousness,his upper rooms by injustice,making his countrymen work for nothing...Does it make you a king to have more and more cedar?Did not your father have food and drink?He did what was right and just,so all went well with him. He defended the cause of the poor and needy,and so all went well...But your eyes and your heart are set only on dishonest gain,on shedding innocent blood and on oppression and extortion. Jer 22:13, 15-17