did they change plugs??? and if the cat was going, one way to tell is to run it hard, then get out, that smell like something is burning up is your cat going bad, if no smell then its not, also the cat can make a humming sound, to me it sounds like a fuel pump prime kinda, if thats a sound you have, your cat needs replaced or if emissions laws dont stop you take it off and get an 02 cheater from
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