Omnibeam Servo motors issue?

I have been refurbishing this Omnibeam laser I picked up and all was going well until this afternoon. I was leveling the bed and while moving the gantry to the far side of the cutting area all of the sudden a horrible harmonic vibration started happening. I never noticed this on the near size of the cutting bed but anything towards the rear of the cutting bed has this horrible noise which is worse the faster you try to move the gantry. I have inspected everything, there is no binding, no obstructions nothing at all that I could find which would cause this. I am assuming this is some kind of imbalance between the two servo motors A/B Y axis drives? I really do not know where to begin with this issue. I was told that belt tension is important and I agree but I do not have a belt tension meter at the moment and just the good old by touch test is how I tensioned the belts up. Maybe this video will help you see the issue. The father back I take the Y axis the louder the noise will be and it will not stop until I hit the E stop to shut the machine down. Any tips to get me going in the right direction would be great.

https://youtu.be/Z4AbOyGq-rI

Here are some pictures of the Y axis drive setup.

Thanks
Mike

Here is another video where you can here the harmonics... This is the servo drives making this noise and like I said it can get wildly loud if you take the gantry back any father. No clue what direction to go to solve this...

https://www.youtube.com/watch?v=npi-hAHOG5I

I seem to have solved the majority of the issue by getting the belt tension closer to what I assume is correct. The issue is still there when it is moving at rapid acceleration on the back side but not near as bad as before. Here is a sample of the machine cutting some test files, it moves around pretty quick and this is not really a high speed setting.

https://www.youtube.com/watch?v=bgy-3WWN4Sk