Steady tapping from passenger side valve cover. Increases and decreases with rpm. Sounds like a lifter/ hydraulic tensioner but not certain. Could not find a write up on this . I am unfamiliar with the 6th gen mustangs as i have only ever owned 5th gen. I am a resonably talentented mechanic. Just need to know how difficult the job will be before i tear it down as i need to accomplish the job in a single weekend even if every possible thing goes wrong. (Or if i am way off base and this is a common issue with these models that is as simple as needing an oil change.)

 

They do sell some lifter "quiet" stuff you can try. Says to put in the crankcase till the next oil change. If noise persists the you probably need to tear into it.

 

Could be a Cam Phaser.