I had this once. It's likely that the brake booster diaphragm is leaking internally and pulling air through the booster diaphragm. I had this happen once and it was bizarre. Nobody mentioned it as a possibility.
For me, it sounded like water flowing through a showerhead.
The brake booster check valve is to keep vacuum from flowing from the booster to the engine and tampering with vacuum readings. It doesn't really do anything to help the brake booster work better, so it failing wouldn't be noticeable.
It could be a failed seal on the brake booster where the line goes into it, but I would bet on it being the booster diaphragm.
Good luck!
Will
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