DIAGNOSIS Yeah, that's usually fine, but it's annoying. The noise is the weighted regulator dancing on top of the vent pipe as steam escapes. It can chatter if the steam flow is very vigorous or if the weight isn't sitting perfectly flat.
FIX First, make sure you're using the correct heat level. Once the regulator is rocking steadily, you should reduce the heat to the lowest setting that maintains a consistent, gentle rock—about 1-3 rocks per minute. A frantic, loud rock means your burner is too high. Turning it down will often quiet it significantly.
IF THAT DIDN'T WORK The three-piece regulator weight (the one with separate 5, 10, and 15 lb pieces) can sometimes vibrate apart. Make sure all the pieces are screwed together tightly. Also, check that the vent pipe (the post it sits on) is clean and straight. A bent pipe can cause an uneven, noisy rock.
HEADS UP Don't be tempted to hold the weight down or muffle it with a towel—it needs to move freely to regulate pressure. The noise is normal, but if it suddenly changes to a hissing scream, that could indicate a clogged vent pipe. In that case, turn off the heat immediately and let it depressurize completely before investigating.