WHistle after head gasket replace

My son was told the solinoid problem was a leaking head gasket which I disputed. But he had it replaced anyway. The mechanic said he fixed the starter, covered in oil and cleaned the oil off the engine and spray cleaned radiator. Please note we haven't had any problems, just had it inspected and was told the engine was great. Now son says the vehicle is making whistling sound that gets louder when he drives. Isn't this probably the exhuast manifold. Or what else could it be?