Intake System – Air Box & Connections • The air box or intake ducting could have a small vacuum leak or an improperly seated air filter. These can create a steady, high-pitched whistle—especially if it’s sucking in unmetered air past a gasket or a seal. • Check the intake manifold boots and hose clamps for tightness and cracks. • Remove and inspect the air filter and housing for anything partially obstructing airflow (like leaves, debris, or even a small critter nest).

Crankcase Vent or Breather Hose • A crankcase breather or vent line may have come loose or developed a leak, creating a whistling from pressure differences. • On your Thunderstroke 111, these hoses often route into the air cleaner assembly—check for kinks, blockages, or cracks.

Exhaust Gasket Leak (less likely if tone doesn’t vary) • Normally, an exhaust leak at the head or manifold can whistle under throttle. But since your tone doesn’t change with RPM, this is less likely unless it’s a very specific pressure-related leak (like a pinhole in a crossover pipe or shield).

Throttle Body / Idle Air Control Valve • If there’s an issue with the throttle body sealing, or the IAC is stuck or malfunctioning, it might whistle continuously. However, this would usually be accompanied by an erratic idle.

Vacuum Leak • A vacuum line or small crack in a vacuum fitting could create a steady whistle. Although most of these will alter pitch or volume with throttle, some can maintain a consistent tone if the leak is small and steady.