2002 Chevrolet Cavalier Tapping Noise

Sometimes when I start my car there is a loud tapping or clanking sound coming from the front of my car. As it idles it will quiet down but it's still there and it sounds like a rattle or tapping. The loud part doesn't always happen but when it does it's when I first start my car. It only happens about 30 percent of the time. When I accelerate the tapping speeds up and then slows down when I release the gas pedal. I just had an oil change and the serpentine belt was replaced but the tapping still happens. An oil additive was out in and the engine lubed. What could the problem be?