If it really is at 1/2 engine speed its probably a hydraulic lifter that's failing. The only parts that run at 1/2 engine speed are the cams and the fuel injectors.
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2004 Boxster S Silver - FUNTOY
2002 Boxster Base Guardsy Red - FUNBOX
1987 Caterham Super 7 1700 Supersprint
2009 Mercedes Benz CLK 350 convertible
1941 Dodge Luxury Liner Coupe
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