2011 f150 lariat 5.0l, whistles under slight acceleration "pedal 1/4 way pushed down" but then dissipates when you let off gas or push pedal all the way down. Stock intake system, air box, etc. Checked all vacuum lines and air hoses I could find. Pcv valves and hoses fine. Does not whistle at idle, only under load.
Check for a cracked manifold or cracked flex section at the manifold
it is under load so it is not a vacuum leak as the vacuum is low under acceleration and high under deceleration or idle
indicates a gas leak or possible blockage from failed cat converter
SOURCE: 2001 pontiac montana
If it is not in the engine, it could be the windshield. We had the same problem with a leak in the gasket of the windshield that airflow caused the sound. Took us forever to figure it out.
SOURCE: My 1994 Jeep Grand Cherokee
If you can get it to whistle at idle, use a peice of plastic tubing, say 1/4 in about 4-5 ft long. Long enough where you can hold one end to your ear (not IN your ear!) and move the other end around your engine compartment. Do some 'sniffing', where it sounds the loudest will be close to the area where your leak is. Replace any hoses that look 'iffy'. Hope this helps.
SOURCE: brake pedal pressure "whistling" whether totally stopped or braki
There is a seal where the rod from the booster connects to the foot pedal that is probably leaking
SOURCE: 2002 merc cougar 2.5 liter has loud whistling
Sounds like a vacuume leak, use some carb cleaner in a spray can and gently spray the engine. When you find the leak the engine will stumble, check in that area for the leak.
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