2004 Nissan Frontier Whistling noise

Hi. When I start up my Nissan Frontier, I always hear a high pitched whistling noise. It sounds like teapot when it is heated high. I hear that sound for about a few seconds and then it stops. When the weather is colder, like in the evenings, I hear that whistle when I am starting up my truck but the sound continues for a minute or so. Sometimes, the noise even continues when I drive a short distance. Also, when I am stopped at the stop light I hear that whistle when I am changing gears or accelerating. Yesterday, I took my truck to a Brake Shop and the mechanic looked at my belts. He showed me that he was looking for frayed eyes on the belt or any fissures. He said that he did not see anything wrong. He said I could drive my vehicle without immediate repair. This has been ongoing for at least 8 months. Please help!