1998 Toyota Corolla NOISE & VIBRATION FROM REAR OF CAR

GREETINGS, TOYOTA LOVERS! WE HAVE A VERY RELIABLE 1998 TOYOTA COROLLA 4-DOOR SEDAN WITH 92000 MILES. WE ARE HEARING A RHYTHMIC NOISE AND FEELING A VIBRATION AT THE REAR OF OUR CAR WHEN WE DRIVE IT. THE RHYTHM OF THE NOISE IS SLOW AT SLOW SPEEDS, THEN FAST AS WE ACCELERATE, ACCOMPANIED BY SOMEWHAT OF A VIBRATION FROM THE REAR. THE TIRES SEEM TO BE O.K.----NO BUBBLES. WHAT COULD THIS BE? IS IT AN EXPENSIVE REPAIR? THANK YOU FOR YOUR SUGGESTIONS.