There are four orange and black rubber inserts that the hard drive sit into. one side has full circles and the other side has half circles. ( The other side of the half circles are located on the hard drive bracket that was previously removed).
The new upper case may not have these inserts. e sure to remove them from the old and insert into the new.
The inserts are easily pried out with a spudger or a flat tipped screwdriver. They are not glued in, but instead have notched sides to hold them in place.