if there is excessive air "whoosing" noise when they are spooling the turbos could be on their way out. I know when I had my old s4 turbo on it sounded like it was spooling 16psi but it was only doing 6psi. There was also little deposits of oil and with all my emissions removed there was no way for the oil to get in the cold side except through the seals. Usually the twins crap out around 100k miles if properly used, i.e. boosting the crap out of them