I have his same derailleur . There is a spring that puts tension on the derailleurs body . It is inside the housing that the main bolt goes through that holds the derailleur onto the frame . The spring they use is to weak and it doesn't allow the body of the derailleur to be pulled away from the gears . In no time at all the body falls down and the cogs rests on the gears and causes the bumping . The only thing you can do to fix it is to drill another hole more counter clockwise than the other one so when you put it back together and turn the B screw housing counter clockwise it will have more tension on the body . Watch this video starting at 7:40 where he starts taking the weak spring out of the housing . Lets say the body notch and the B screw housing notch once put back together is in the 12 oclock area and the hole in the B screw housing is in the 4 oclock area . What you want to do is drill another hole in the 3 oclock area so when you are turning it counter clockwise to put it back together the spring is getting smaller and it will be putting more tension on the spring when you are putting it together . This spring tension is the only thing that keeps the main body away from the gears on the rim . You may have to drill the hole back to the 2 oclock area if the 3 oclock area is not enough . This video is for the R 3000 but it works the same way . Notice when he puts the derailleur back on the trainer starting at 15:08 you can see that when he pushes the body up it springs back down . That is what you want to happen . If that doesn't happen once you put the chain on its going to raise up the body again and its going to cause the cog to hit the gears again .