The noise was a hiss-like sound

I recently had my brakes and rotor replaced and the first time I drove my car after the new brakes were installed, I had an unusual new noise when driving my car. At first the noise only occurred when I was slowing or coasting to a stop. The noise was a hiss-like sound. I called my mechanic and he was not concerned, but I did schedule an appointment to have the car looked at/repaired. In the meantime, the noise has gotten worse. I have limited my driving since receiving the new brakes and a new noise, but did freeway driving this weekend. I am no longer driving for fear that driving may cause more damage to my car or leave me stranded in one hundred plus degree temperatures.