Noises problem
1988 Other Nissan Models 4 cyl Front Wheel Drive Manual 225000 miles

Recently I replaced my break pads, after that, there has a whistling noise coming from the front of the car, only on and off and after sometime it goes off. This also happens when is being driven for long hours. I have shown it to my mechanic as well and he has no clue, what it could be, he said that, he needs to hear it, for him is diagnose the problem, but when drives around (short time) there is no noise. What can it be?