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my car has 150,000+ miles on it. I have performed all maintenance required for that high mileage. upon start (but not EVERY time) the car makes a very loud knocking noise that continues sometimes all day. The noise occurs at idle and at accelaration as well.(altho when driving at higher speeds its harder to hear) The noise does not occur all the time-but its now increased in frequency. I drive in commute traffic (sf bay area). Maybe unrelated-but the check engine light goes on and off. I have recently switched fuel grades and that seems to have helped that issue. Any clues?