Knocking sounds while braked from my 08 Chevy Cobalt

4 months ago I bought a former rental car, a 2008 Chevy Cobalt, with 39,000 miles on it. 4 months and 2000 extra miles later, I have my first complaint. I find that when I come to a full stop and hold down the brake, I sometimes hear a faint then eventually louder knocking sound from under the hood. It almost sounds like a little drummer going to town on a piece of metal, but man does he keep the beat. The noise immediately stops when I release the break, and the sound only starts when I am at a full stop. Anyone?

My mother-in-law’s Pontiac did the same thing and it turned out to be the antilock brake system needing adjustment. Definitely something to check out.