Does anyone know what would cause a ringing noise while driving?

After I took my car to Midas to have the brake pads and rotors replaced and an oil change I noticed a ringing noise. I immediately took it back and they said it would go away after driving about 20 miles and it was probably just the rotors adjusting. Two weeks later I took it back because the noise is still there and they can't figure out what is causing it and don't think it's anything they did. The ringing sounds like a bicycle bell or maybe metal rattling and becomes really consistent at speeds above 45mph. It's hard to tell where the noise is coming from but I think it's the steering console or dash. It's so annoying! Has anyone else had this happen?