2001 Chev Impala base (47K).

Only when accelerating, I can hear a kindof air sucking noise from front.
Getting onto highways, it's much louder with hard acceleration, sounds like
metal scraping.

The noise has been there for a few months. Got the car to two chev dealers
and another mechanic. Vacuum leak, cat converter, etc...etc... checked
triple times. They told me "everything possible checked", but found nothing
wrong. It's just ridiculous. The car runs ok, but with the noise came up,
acceleration seems not as powerful as before.

Thanks.