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Whistle Sound 89 Bronco

Hi–I have a 89 Ford Bronco XLT 5.8/351. It started whistling yesterday(like a tea kettle) through the dash. It lessens/disappears ONLY when I accelerate. It does not seem to be affected by braking and, it gets loud when I am idling or when I decelerate. It is driving normal otherwise (oil pressure normal, running cold, starts right up, no rough idle, no hesitation)–any ideas? Doing some research points to a vacuum leak or brake booster. I had the brake booster replaced 6 weeks ago. I lifted the hood, and listened at each footwell inside the truck–can’t pinpoint where it’s coming from. Again, it is driving fine at this point, but the sound just started. I know I am probably going to have to take it to my mechanic, but I like to have an idea before I take it in. Thanks in advance for any advice.