Sounds like its the records or something in the coils or connection to them. That is strange that you don't here anything when the stylus is off the record. How old is the cart? I know I get hiss from mostly head shell lead connections, or that one of connectors on the back of the cart has come loose or died (happened twice).

Could be that diamond is fractured from age or life and that could cause much distortion, especially approaching the inner groove.