This is our first camper with a slide. Should be slide be flush with the floor when all the way extended. Also, is it normal for it have a sort of grinding sound when it reaches the full extension?
Not positive on the newer slides, but they should be pretty flat when extended. Mine is with the only lip being the thickness of the carpet. As far as the "grinding" sound, that may be the clutch mechanism ratcheting to indicate full extension. If it is, then the sound is normal but release the switch as soon as you hear it.
Ours has small lip - thickness of carpet and plywood subfloor.
This is normal I would think. Straight in and out is a lot simpler that out and down and up and in. Not saying they don't exist but I have not been in any that were flush when open.