2004 CLK320 (M112) water pump replacement
 

The pully on my water pump seems to be running out of true so serpentine belt is catching the engine casting.
3 Questions.
1) How do I take the tension off the belt so I can remove it and investigate problem?
2) Is the water pump running out of true a common problem?
3) Are there any particular issues to be aware of when replacing the water pump?

Any help gratefully accepted, thank you

Well if you have to ask how to loosen the belt tension perhaps this is not a job for you. It may end up costing you much more if you mess things up.