vic making a whistling noise under hood when running, changes pitch , does not affect performance

Hi, this is Bo, an auto expert on RepairPal's Car Genius team! A whistling noise under the hood of your '99 Crown Vic could mean you’ve got a vacuum leak playing solo in your engine bay. Vacuum leaks often create a high-pitched whistling sound, especially when airflow shifts as you rev the engine, and they don’t always mess with performance—at least not right away. Start by inspecting vacuum hoses and fittings for cracks or loose connections. The PCV valve is a common suspect too—it might be worn or clogged, causing air to whistle like a tea kettle. If you’ve checked hoses and still hear the sound, the culprit could be the intake manifold gasket or throttle body gasket letting air sneak in. Spraying a little brake cleaner around suspect areas while the engine idles can help pinpoint the leak (just listen for changes in RPM). If nothing obvious shows up, it might be time to have a shop do a smoke test to find the leak. If you need any further advice or want help finding a trusted RepairPal shop in your area, please feel free to call us at 877-913-1554.