Tightening belts causes y-rail noise Vcore3

Has anyone had this happen?
After getting my deracking fully sorted with belts off, and the y-rail sliding nicely even with some inertia, I tighten up the belts and now its movement in Y is clunk clunk clunk clunk.
Movement in the X doesn't have this problem, therefore limiting the variables to the Y rail bearings.
Any thoughts?