I've found that the x idler pulley tends to ride up against the top y carriage wood. This creates noise and other problems.

There are bearing and belt holders and all manner of x axis improvements but I've yet to see one to fix this. The idler rides up due to the belt being pulled up to mesh with the x carriage belt teeth.

My solution is to customise the teethes position to match the belts natural path.

This is based off of Joakim's x follower openSCAD file so it can match its customization for a perfect x axis sliding action, print his and mine as they replace 2 separate parts.

All comments and suggestions welcomed, heck just comment

BTW: http://www.makerbot.com/blog/2012/02/07/x-buckteeth-leader-by-timmytool/