The Y axis nut mount on these mills is bad for working loose. They only have one bolt and very little surface area to give stability. You will have to go from the top down so no need to remove it from the tray. Just remove the table and go from there. When you unbolt the one bolt clean everything real good (carb cleaner spray or paint cleaner) and use blue Loctite on the bolt when you install it.
Many people have remade there Y axis mount to include a extra bolt but this will require doing some mill work on the bottom of the saddle where the nut mount fits. Well worth the effort if you can get to another machine to do the cutting. If not using the Loctite will help a lot.
Jess
GOD Bless, and prayers for all.