Inspect your vacuum lines.
Your Heater controls vacuum reservoir will look like a black plastic sphere, mounted to the top of the PS inner fender liner. Pretty much directly under and forward of the air filter box. Give those two lines to/from that reservoira good look, they tend to crack in half and whistle. That would be a vacuum leak.
It should also affect the operation of the vents in the dash, making them only flow air out of the defrost. That's my best guess without being able to hear it, but it could be any of the vacuum lines.
2000 2.5 MO5D 4.10 one-wheel drive DD - gave to my older son
1990 STX 87 2.9 going in.....Junked it
2000 3.0 4WD M05D 4.10 S-cab Off-Road package with 235-75-15" tires, hotrod DD, lol