1997 Ford Explorer '97 Explorer high pitch whisling sound w

Hi,

I just bought a '97 explorer V6 4.0L SOHC a few months ago, with 133000 miles on it. The last owner took good care of it and everything seems fine until a few days ago, when I start the car and step on gas, there's a high pitch whistling sound. It only happens when I accelerate and running in low speed like <30mph. If I give more gas and rpm goes up, it stops. But if I release the gas and then give it a little gas again, the sound comes back. And after running a few minites, the whistling sound disappears totally, everything back to normal.

When the high pitch sound happens, all readings from panel seems normal, the RPM is only about 2000 as regurlar, and the last owner just changed the brakes a few months ago. I live in Georgia, so the weather now is fine. As a stuggling student, I really appreciate some advice before I go to the repair shop. Thanks!