I've been trying to figure this out for quite some time. I don't think it's a warped rotor because I cannot feel it in the pedal or steering wheel. The car does not shimmy or shake. It's a pulsating sound like a kind of groan or grinding sound that pulsates as I brake down twisty hills at about 50mph or faster. I don't hear it on straight or flat surfaces such as stop and go braking in town. This would be heard on long braking events like down a long hill, especially one that twists and turns. Two friends suggest that it's not warped rotors since there is not pedal pulse. They suggest perhaps bad pads, or a glazing imperfection on the rotors. I can't tell which one, but it feels like one or both front wheels. I've heard rotors can get glazing spots which may make people assume warped rotors, but it's not. Any ideas on troubleshooting this? I don't want to blow money on new rotors or even pads. Perhaps just getting the rotors turned and seeing where that leads? Any advice is greatly appreciated. JG.