If your vehicle is showing loud whistling or siren-like noise when accelerating, here are the most likely causes and the parts you'll need to fix each one.
A glazed drive belt or a failing bearing in an idler or tensioner pulley can produce a high-pitched whine or whistle that increases directly with engine RPM.
Unmetered air being sucked through a cracked hose or a failing intake gasket creates a high-pitched whistle, especially noticeable when accelerating or opening the throttle.
Air escaping from the pressurized intake tract under acceleration causes a loud whistling or rushing air sound. This frequently occurs after recent engine work if hose clamps are left loose.
A failing bearing inside the alternator, or an electrical diode issue, can cause a distinct siren-like whine that matches engine RPM and may worsen under heavy electrical load.
Worn bearings or damaged compressor wheels in the turbocharger create a classic 'police siren' whine that rises and falls with engine RPM and boost pressure.
A small crack or blown gasket in the exhaust system can force exhaust gases out under high pressure during acceleration, sometimes creating a whistling or squealing noise rather than a typical exhaust tick.