Liz, does it not just need a good clean? You can buy cleaning tapes. Try these. If not, you may need to pay for it to be cleaned. (Someone will take the lid off and clean it with cleaning fluid.)
Rented tapes tend to be quite worn. As they play they shed dust from the tape on to the machine's parts. This dust can be compacted by the rollers to make the workings run in a lumpy way. I suspect this is what you are hearing.
Sarah Lincoln
October 2003