air pressure

I experienced this problem. The dealership said if it was the universal it would be covered 100% and we would not pay a dime. They checked it out and about 3 hours later told us there was absolutely nothing wrong, charged us $150.00 which they said was only for 1/2 the labour costs. ($100 x 3 hrs). I was furious. We drove home and have not had that humming since. I believe it was the universal but the dealership made more money by saying there was nothing wrong. If the warranty had covered it, we would have paid nothing.

a universal joint would more likely be a squeak or squeal like metal against metal and not really a hum where exactly does the hum sound like its coming from?