A whirring sound during Gantry Squareness check

Batch 1.2- I have finally completed the upgrade to Gen2+INDX last night, but I am having trouble with the Gantry Squareness Check. Initially, my gantry was skewed by approximately 150mm on the left side, but I took the tension screw out that holds the belts, slightly bent the gantry, then tightened the screws back. That seemed to fix the gantry not being flush with the pulley housing in the back.

When doing the check there is a high-pitched whirring sound. I am having trouble pinpointing what that means. Pulleys? Belt? Gantry? Please refer to the video for the whirring sound.

Thanks for any responses!