Noise in rear wheel left side

A noise has developed in the rear of my van, left side. It sounds like it is coming from the rear wheel. It is a noise like that of a flat tire, but doesn't happen all of the time, it does not interfere with steering or braking. It then becomes a vibration noise and I can feel the vibration when driving. I had the service station put it up on the hoist and he could find nothing wrong. Everything was tight and appeared to be in good shape. However, the noise is getting more persistant. Any ideas what it could be?