Me and wife just bought a 2014 audi s4 dsg, stage 2 apr supercharger pully, exhaust system was completely stock with cats, car was drinking coolant and a bit of oil, (only had for about 1000 miles), missfired on 6,5,4 and the engine was making a tapping noise, and the car completely had shut off, constant missfires, had it towed to a shop that works on these cars, Basically, I was missing on 6,5,4, driver side bank, and there was a noise from the engine, started it up after pcv replacement, seals etc, car was still missing and still making that noise, They started tearing back down, compression tested those cylinders, compression was 130/130/129 on 6,5,4, he said he unhooked the o2 sensor and the car ran with no misses even when revving, they boroscoped the cats and they were melted inside, very clogged, badly, with the o2 sensor unplugged, it still has this noise but no missfires, but as of right now the exhaust is only exiting the small bung hole for the o2 sensor, and the mech thinks that it is still pushing back up into the engine, and that is the cause of this noise.

Looking for other opinions on what this noise could be, will be having him take the cats off regardless.

https://youtube.com/shorts/UER2z5nl9Qo?feature=share