Pop the bike in neutral and let it idle...pull the clutch in, let it out and see if that isn't the noise...clutchy things can make a bit of noise on these things, so I keep getting told...I tend not to notice
The tensioner should be on the front of the engine, to one side I would assume. A couple of bolts with a round thing in the middle. Gee, that description narrows it down, eh? Maybe someone else with the same/similar bike can take a photo for you so you know what your are looking for.
Wouldn't expect a lot of slack at 25,000km though...why I think it may be something else...

But I guess the tensioner could be doing something weird...
man I need a coffee...