Over the past week, I've noticed a substantial "chattering" sound emanating from the passenger's side of the engine. By listening closely, it almost sounds like it originates from under the valve cover. I thought maybe it was detonation, but it is even present at idle (I've always noticed chatter under acceleration on steep hills, so I run 89 octane to lessen the pinging). It could also be coming from the alternator (maybe worn bearings), but I'm almost certain that it's coming from the engine. The car doesn't have a miss, there isn't a power loss, and the chatter increases when I bump the throttle. What could be wrong?