my guess would be that you have an internal engine un-ballance that exites the engine in a resonant frequency at the stated rpm... the time delay could be from a failed engine mount thet took 300 miles to surcome to the beating at 3000 rpm...
the diagnostic fix is to replace the engine mounts.. and the long term one is to remove the engine and have the entire rotating assembly (including flywheel and clutch) dynamically ballanced... (if you arent running an incorectly ballanced damper or flywheel in the first place... they could be labeled wrong, or somone have given you an internal ballanced when you needed external or vice cersa)
my 2 cents
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Elwood:We're 105 miles from Chicago, we have a full tank of gas, half pack of cigarettes, it's dark out and we're wearing sunglasses.
Jake: Hit it.
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