What would be the cause of a rear wheel noise? It is getting louder.

Tires 'cupping' on inside of tread , probably. Have the tires been rotated regularly? Has it had an alignment? Condition of struts? Tire balance been checked? All could have an effect on tire wear. No fix other than replacing tires , if wear pattern is the cause of noise. If you replace them get the suspension and alignment taken care of as needed , so as not to have the new tires do the same.

POSS TIIRE NOISE OR REAR BEARINGS