I bought a catalytic converter less than 2 years ago and already its rattling. I have recently replaced the valve seals and the valve cover gasket, and I am going to have the fuel mixture checked when I replace the cat. Is there anything else I should check that might have caused it to fail so quickly? Car is a '90 560SEL. Thanks for any input.

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90 560SEL
H&R springs
17" Alba Frontiers
Cross drilled rotors
Magnaflow Cat
European Headlights
WALD front bumper
Keyless Entry
Burlwood shiftknob
Clear corners, taillights
Limo tint
2 JL 12W6's, JBL BP1200.1, Pioneer P8200R