hi guys..i went to the trouble of replacing the aux belt,tensioner and pulley for the upgraded kit on my lovely x type...
its rattling again.
i had a thought...what if i moved the alternator back a few mm with washers...therefore tightening the belt slightley and marginally compressing the tensioner...might eliminate the slap/rattle at low rpms..no 7 is the alternator

 

Some ford owners have fitted a clutch pulley to their alternator and reduced the rattling this way. I don't know if x-type already has a clutch pulley on alternator or not. Might be worth checking.