hello, 05 quattro v6,purchased at 50k,exhaust tone and mileage went way down after about a month,started hard shifting and hesitating,i thought it was a transmission problem,took to local trans shop they said -it needs a rebuild for 7k! could not supply a specific explanation -worn out on the inside was all i got from them-but could not tell me what was worn.i started it today and it was running like absolute crap at idle and 40-50 mph baaad hesitation ,i swear it sounds like bad catalytic convertor. at idle it was clearly almost backfiring -sounded like inside the cat,low sound it has done this several times,i had it at audi for a service and check out at 55k they did a tune up ,can anyone help me out?im a single dad with no income -is it the cat? already? and shouldnt that fall under the 80k federal catalytic warranty?thank you all