Check oil is full or above,Check for loose shields around the catalytic converters, the bottom one came loose on me last week. Made a nice irritating rattle at 3000 rpm. The shield can be clamped in place with a couple of long hose clamps, temporary with wire.
My sister's Mystique has a slight knock I never identified, 100,000 miles later, it's still the same. Push in the clutch, does the noise change?
