99 Tacoma, slight whine

I have a 1999 tacoma with automatic and only 60k miles. Anyhow, it has started making a barely audible noise like a high pitched whine when engaged. No sound is made while idling at any rpm, but only as movement starts. however, it is more noticeable when putting in reverse and even slightly moving. As soon as brakes are applied it fades out. It appears to be coming from under the hood (front part of vehicle somewhere). Thanks for any ideas!