My vote is with the timing belt tensioner. It would make sense that the noise is more noticeable at idle since the engine isnt pulling on that belt as much. I can imagine that belt hitting the cover can make a noticeable thud! I certainly wouldnt drive it and only run the engine for diagnostics only. It only takes that belt to jump a tooth or two before your banging rods on these interfereance engines (if you havent already). Check out this video. Its not the same engine but you'll get an idea on whats going on underneath that timing belt cover. Just an fyi i dont think there is a way to take off one of the covers to check without ripping out the fan and pulleys. But i can tell you it will take you a day to replace the timing belt! Good luck! http://www.youtube.com/watch?v=2iafCTng7X8