Slicer 827a roaring noise - Berkel 827E Gravity Feed Deli Meat Slicer

This can be due to the motor bearings/bushes that have worn out and is slightly having the gap within,
When the load comes on the motor must be firmly moving on the main shaft without a fraction of a gap.
To confirm use some light oil into the motor shaft where the bearing/bushes are and try out. If so you might have replace them to fit well , get help from a machine tool shop.
If the mtor seems perfectly alright then the mechanism of the cutting blades, the moving parts here- one of them - can be the suspect. Take carre to check when the power is on.