Could be a number of things. My 2000 S has exhaust resonance at the same rpm, but then I am running no cats and 2" pipe from the after-market headers to the muffler so it is to be expected.
The first thing I would look at is the exhaust. Maybe a part of it is touching the body of the car??
I would not think it is an engine mount, I replaced mine last week and am familiar with symptoms, but I could be wrong.
Good luck with your diagnostics.
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