The rattling is most likely the plastic post the screw went into that snapped off. I've seen several different items made of plastic where this happened. The tech should have suggested it as it is not at all uncommon for the posts to break due to several different factors.
If you use a flathead screwdriver and put it under the stripped out screws and lift lightly while unscrewing, they should come out. If you return the tv and have trouble with the new base, the same technique should help you to remove those screws. Some products seem to be made with what I'll call a softer type of plastic than others. These are the ones I've had issues with several times.
Good luck and Happy New Year
Jack