You should hear the motor running, you can hear the sides sliding against the rubber, you hear the floor rubbing as the slide is moving, you may even hear some minor popping and cracking sounds, flexing of everything. Once it's completely in or completely out, it may go "bang, bang, bang, bang". That says it's done and normal operation.
I suppose "noisey" is a matter of opinion.
What you could do is have the RV salesman demonstrate opening and closing slides on another unit, including a trailer and a 5er, and another motor home. Then you'll be able to tell immediately if the unit you are interested in is normal or abnormal. That's what I'd do.