I have a 2005 Boxster S with 48,500 miles and recently have had a rpm dependent rattle/clacking engine noise develop. It makes the noise constantly does not go away when engine is warm and the noise is there when clutch is disengaged and not in gear.

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In park, or when driving? Inspect the exhaust system. May be a baffle in the muffler.

- Nick

Thanks for the response. Noise is there both in park and when driving. It either goes away or is covered by regular engine noise at high RPM (above 4K). I will check the muffler.

Do not think this noise is trivial.
Use a mechanics stethoscope or long screwdriver to precisely locate the part of the engine making the noise. Guessing just produces a long list of worries.
Do not drive the car until you have located the noise and diagnosed the problem.
Many noises that sound the same have totally different locations/significance. For example, what sounds like lifters could be a rod bearing going. Bad lifters & the beginning of rod knock are indistinguishable at some stages -but a stethoscope test will quickly locate the problem.
If all else fails, drop the pan and look for bearing debris.
PWIS?
Let us know what you find so we can help others in future?