I remember a few posts in tha past, where someone had a sound that was like clatter or even the dreaded "death knock"...turned out to be the timing belt tensioner saying goodbye, which would also explain the possible circumstance of the timing belt skipping a cog...
In any event, with sleuthing out the solution.
is the timing belt tensioner inside with the timing belt? or is that the same tensioner pulley thingy on the outside?