 Sometimes it's coming from outside the vehicle. This is a weird story about one of my company trucks. We had a whistling coming, and we chased it for years until we cut a muffler and had to put a muffler on it. And the welder that had welded the muffler had ran a little bead of wire up in there inside the flow. Oh, wow. And that was making this whistle we could hear. Wow. And once we cut it out and put a new muffler on, it was perfect.