i have a 2007 model accent gle the car has done 12000kms

i have a 2007 model accent gle. the car has done 12000kms. recently while driving at a speed of 140kms/hr i heard a hissing noise contineously & start driving at 120km/hr after 50kms i again heard the same noise. it seems as two steel parts are rubbing each other. After that i stop my car & went to a highway local mechnic he checked the car by removing wheels & told that the problem in not in brakes & the alignment is also proper. I have to give my car to service center but want to know what can be the problem. Pl advice