[Duncan]

You can adjust the belt by loosening the big nut on the tensioner at the front of the engi...

You can adjust the belt by loosening the big nut on the tensioner at the front of the engine and then turning the long adjuster nut at the top of the engine.
If the belt is cracked you might want a new one.
I have read other people say that the tensioner can stop working so if you turn the adjuster and nothing happens that is your problem.

There is also a shock absorber that stops vibration in the tensioner and when these wear they rattle.
You might be able to see it rattle in Drive with the handbrake on.

 

[Duncan]

You just undo the big nut at the front. Then turn the adjuster one way for tight the oth...

You just undo the big nut at the front. Then turn the adjuster one way for tight the other way for loose. You should feel it getting tighter