I bought a used Canon 17-40mm F4L a while ago, and I just noticed it had the Zoom Ring kind of loose and also a lot of my pictures that were taken at the widest were 19mm instead of 17mm
I am answering my own question since I managed to fix it myself, but wanted to share it with other people who might have the same problem!
It is actually quite easy, all you need to do is to pull/slide on the rubber of the focus ring (it's not clued) to reveal what is under it and you will find 2 screws which might be unscrewed... just screw them back and should fix your problem! no need to send it to Canon and will not void any warranty.
Unless you've ever been inside a lens and know what you are doing you are much better off sending it to a Canon Service Center for diagnosis and repair. IF you take it apart first and can't get it back together it will cost even more to have them fix it because:
1) You'll probably wind up damaging something else that wasn't broken taking it apart.
2) They charge a "tamper fee" if they'll even attempt to repair an already disassembled lens, and often they won't even attempt to fix it at any price.
For a more complete warning list of why you shouldn't work on lenses yourself, see Roger Cicala's blog entry about cleaning dust from the front and rear elements of some of the easiest lenses to service. Getting down into the helical zoom collars is several levels deeper than the front and rear optical elements on most lenses.