Well, you say a loose chain. But, a lot would be visible by looking at the chain. Hanging slack? Rear derailleur when the bike is shifted into the big ring up front and the big sprocket behind. Also the rear derailleur when shifted to the small ring up front and the small sprocket behind.
A clunking feeling might also be due to a sticking freewheel or freehub. That might be difficult to photograph... But you could backpedal and see if you get more slack in the upper part of the chain. Also being slightly out of gear, or badly cross-chained can cause that same slack feeling.