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This may be more of a general automotive question than a Infinit-specific question.  My 2003 G35 sudan has within the past week, started to shake noticeably once i reach 60 MPH.  My car has just over 50,000 miles on it and has always run smooth as butter.  I'm very ignorant of the inner workings of the engine and cars in general.

I have over the past 2 weeks have gotten a synethic oil change and rotated my tires.  I'm hoping someone can help me with this problem or provide some possible causes and/or remedies before i take it into the dealership to get fixed.

Thank you very much,
Ty