Cat 257B MTL bucket dropping

I am beginning to have a problem with my 2004 Cat 257B MTL.
It has happened twice in the last two days.
When I load the bucket and return the control lever back to the neutral position, the bucket begins to dump the material. At the same time, I can hear a squealing noise.
I can raise the bucket up but it will drop as soon as I move control to neutral.
After playing with the control a bit, it will start to work again.
Anyone have any idea where I would start to look for the problem?

Cat 257B MTL bucket dropping