A faulty AOS can also make a whistling sound....
Try removing the oil filler cap while the engine is idling - if the cap is difficult to remove (a vacuum being pulled), it's a good indication you need a new AOS.
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2001 Boxster S (triple black). Sleeping easier with LN Engineering/Flat 6 IMS upgrade, low temp thermostat & underspeed pulley.
2001 MV Agusta F4.
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