To avoid having to pay a lot in the way of vacuum cleaner repair, always practice good maintenance, such as:
- taking time to clean the strings and debris off of the roller brush
- replacing bags regularly
- dusting out the inside of your vacuum
- replacing belts and HEPA filter every 6 months
A foul odor -- that doesn't smell like rubber or electricity -- coming from a vacuum is likely to be pet dander or the like. Throw away the bag immediately, wipe down the inside of the vacuum with all purpose cleaner and replace the filters.