On the top of mxer the security bits can be removed with the proper torq bit. After removing top cover and diaphramn either look straight across the top of mixer horizonally or gently drag a small straight edge across. What your are looking for is in the very center where the cone of diagphramn goes in there is a plastic insert that may be loose or slightly risen. This is not suppose to be loose at all and if so can interfere with flutter of diaphramn. Other than this I would look closely or spray for vac leaks.
Hope this helps