My 2007 S60 2.5T (95k miles) has started making a loud and rapid clicking/knocking sound that seems to come from the engine. The sound is pretty constant when the engine is running. The clicking is more rapid as I accelerate, and slows a bit as the car slows or idles. I haven't missed an oil change. Any idea what could be happening? I'm expecting the worst, but hoping for ideas other than "the engine is trashed." I do have a video, but I'm not sure how to share it here. Thanks so much!