Car makes buzzing noise on 2000 Chevrolet Metro

Car makes buzzing noise when cold. It does this for about the first 15 minutes, especially when accellerating, and then seems to clear up or diminish. Any idea as to what that might be? It's my sister's car, so she says she's had the serpentine belt replaced and some tubes (though I don't know which ones) replaced and no one has an answer. Thanks!

Asked by for the 2000 Chevrolet Metro
First is it a 3 or 4 cylinder engine? How many miles on it. For future help email me at