is the noise coming from the engine something else? if u have sludge build up i would recomend a can of moa44 it will clean alot of sludge built up as long as u follow the directions. if it was your rotors you should probably feel a vibration through the break pedal, if it the noise gets faster with speed i would shut the car off and check to see if the fan is warn out (only when car is off) by grabbing it and seeing if it wiggles if so its bad, i would also check the lugs on the tires ive know of noises like that from the wheel having play from being loose. my gut is saying if it has a helicopter sound it could very well be a loose fan, if it is that and doesnt get fixed it can and will go through the front end ratiator and all and cause extensive damage could also go though the hood ive seen some crazy things happen with cars. but without more info its us just guessing what it probably is. i would not drive it unless it was to the shop or someone that knows there way around a car!!