If they put new guide and chain in it it the drop is probably good but easy to check with a set of calipers. Like stated do not rotate the engine while the centre nut is out. But before you pull it put the bike in 2nd gear and rock the bike back and forward so the engine bumps both ways this will allow the tensioner to adjust before your take measurements.
A caliper it the way to go
make sure you measure to the shoulder edge not down inside where the spring goes. 16 mm or more require a spacer which can be mad out of a 10 mm or 3/8 bolt shank .355" long. it fits in the spring hole and increases the tension on the spring.
Oh and it would not surprise me if your hearing the lifter piece rattle as the 06 had the older style with a smaller hear its not bad it just rattles a little more then the newer big headed style see the comparison below.
The tensioner was replaced as well surely it won't need a spacer already I have measured as best I can it's 20mm