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F100 I Beam w/ straight 6 (original motor) I can drive for 5 minutes sometimes or for 30 minutes before it happens. Its so random, but it always happen eventually.
The truck has its original straight 6 engine with an aftermarket carburetor. We changed the gaskets on the carburetor and tightened all the hose clamps all around. Still there is a whistle. When I have the truck in idle it is very faint so its difficult to locate when i have the hood up. I assume its a pressure thing but two mechanics in my area can't figure it out and I am new to owning a classic vehicle.
A vacuum leak is the likely cause. That brake booster looks new and could be the culprit. Not a lot else to check, carb, intake manifold and distributor advance. The nice thing about these old trucks is that they don't have miles of vacuum hose like the new stuff.
Disconnect the vacuum line from the booster and see if the whistle goes away. Just member that if you drive it like that, the brakes will be MUCH harder to engage.
If the noise is very faint you can make your engine compartment quiet by using a second vehicle to supply vacuum by connecting your manifold fitting (engine off) to the other vehicle's intake fitting with a long piece of hose.
The correct way to find a vacuum leak is to spray a flammable substance such as propane from a mini torch (as taught in school) with a hose attached or even easier a hair spray with straw that has flammable on the label. WD40 would work too. The trick is to get in close to your hoses and intake and anywhere else a leak could occur. This includes the brake booster and base of the carb. Do not spray down the throught of the carb as this defeats the purpose of the test. Only spray a tiny amount in as narrow of spray as possible. If there is a sudden increase in engine RPM or smoothness in running you have found your vacuum leak.
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