My tires have developed an annoying 'squishing' noise when riding. At first, I thought there was tar or some other sticky substance on part of the tire tread (I was riding on pavement). That's not the case. The tires are clean and the same noise occurs on the trail, regardless of surface. If I inflate the tires to 15-20 PSI, the noise disappears completely. Below about 15 PSI, it comes back. I'm guessing there is some movement between the tire and tubes causing the noise??? Is this noise common at low pressure?