1. You're going to have to clarify. What's the tachometer do? Is it a squeak? A rattle? A grinding noise? ONLY in park?
2. Could be the belt, or tensioner, go pull on your serpentine belt and see how tight it is. If you can pull it a good 1.5-2 inches it's too loose.
3. maybe now that it has some mileage on it it doesn't filter as well, less resistance, less "sucking" noise, better performance.
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