I have a very noticable vaccuum or perhaps air leak in my 89 Volvo 240 station wagon. It sounds like a gaping hole in the intake manifold or throtle body. I have looked on several occasions, my brother in law who went to college for automotive tech could not find it. Some hoses were not fitting well and i bought silicon hoses of various sizes to ensure the right fit. They look great but I can still hear hissing from the passanger compartment of the car.
I am assuming that you have had the car for a while. Have you heard this for a while or is it something that has just started randomly. Is there any rough idle? Does it get louder when you accelerate?
I’ve had the car 7 yrs. This is fairly new. Another brother in law borrowed it for a few months since I was saving it for my teenage daughters and not driving it. It was a gift from him.
Follow up: It always sounded like it was coming from the flame trap and I kept wiggling it etc. I finnally noticed that the intake gasket had portions sticking out that were unsupported. I suspect that while checking the flame trap that I bumped the gasket and broke a piece off, but it did not fall away and looked as though it was intact. Changed the gasket, voila.