Yep, did the knock test, although unfortunately the test really only works on structures that are the same, so for example tapping the chainstay if the damage is also on the chainstay. In the area where the crack is, there's not much surface area to compare, so tapping other parts of the structure doesn't really yield results that can give you any useful idea as to what's underneath. I guess in summary I'm saying you need to compare like for like when tapping an area. Hope that makes sense?