Did you mention whether it was proportional to engine speed?
Could it be that the header or header gasket is not torqued down properly and leaking a bit.
From too many years of experience, always consider the most unlikely to be perfectly possible, especially when you've run out of logical ideas.
In light of that you might also want to check for a covertly planted critter whistler or a racing pig taking up residence near by. 
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"It broke because it wants to be Upgraded  "
2012 Porsche Performance Driving School - SanDiego region
2001 Boxster S, Top Speed muffler, (Fred's) Mini Morimotto Projectors, Tarret UDP,
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Last edited by jb92563; 09-18-2013 at 06:08 AM.
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