I had the same problem too. Its your timing belt tensioner or your water pump. Im willing to bet that rattling is your timing belt smaking against the timing belt cover because the tensioner is not providing the tension necessary. But its also a wise idea to change your water pump too. That timing belt tensioner costs about $130 at a Mitsu dealership. Advance Auto (my P/T job has that part for about $60). While you're getting the timing belt tensioner fixed, I would also recommend the following...
-Replace your water pump (its driven by the timing belt)
-Replace your idler pulley
-Replace your belt tensioner pulley
-Replace timing belt
-Replace serpentine belt
I paid about $800 for all that work (parts and labor) with crazy discounts. I was pissed having to spend that kind of money. But id be more pissed off with no car. Do the work. Its well worth it.
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