Rotten smell from 2007 gti 12k miles?

i have a 2007 vw gti with 12 miles currently on it. i began to notice a rotten smell coming from my car at 10k miles. the smell started in the from of my car, then i began to notice it out of my exhaust and the right side of my car. Also now my exhaust makes a pinging noise for several minutes at a clip. The car makes the smell whether i drive slow or fast. and it also does it while idle in my driveway. i run 93 octane or higher at all times and i do full syn oil changes. My first thought was my cat or my injectors. I have had it into the dealership three times. 1st time . they said they couldnt smell it. but didnt test drive. 2nd time they said smelt it but didnt find any fault codes change gas they say( say they test drove mileage didnt change a fraction) changed gas to humor them. put 400 m on the car in 6 days. 3rd time they drive it 140 miles, charged me for the gas said the smell isnt there again. got it back after 2 days smell getting worse, Any Ideas?/ Advice!