Reverse Problem on 2005 Yami 4 stroke Help!

I have a 50hp 2005 Yami four stroke, All of a sudden lately when I shift into reverse there is a knock every few seconds, The boat still backs up but with that knock, I pulled the boat and my friend adjusted the shifter cable, it was way off! Put the boat back in and it works much better but still the knock is there however not as often as before, Forward works fine, Could this just be that it needs a little more adjustment? Or should I bring it to a dealer since it is still under warrenty, I would rather not if I dont have to due to the down time, any tips or tricks or something I may be overlooking would be greatly apreciated, also, I have the original prop which is aluminum..Thanks for the help!

Bill

[boats.net]

Make sure prop nut is tight. You can change lower unit oil and check for water and metal on drain screw magnet, if you find metal contact your dealer before cleaning magnet.