I took the car out today, and as it was warming up in the driveway, there was a pretty loud knocking from the passenger side near where the intake hose is in the engine compartment. After a few miles I stopped for gas, and restarted it with no knock. When I took it home and let it cool in the driveway, then started it up, the knocking was back as I put it in the garage. Sort of three knocks, pause three seconds, three knocks, repeat. Car drove fine all day with no knocking (when warm), and this is the first time it has ever knocked in two years of ownership, even in colder weather.