I have a 02 1.8t with about 90,000 miles. In the past 2 months i have changed my cat. and the thermostat, and the water pump and timing belt about 3 days ago now and now when i start my car it sounds like a vacuum sound starts and goes for about 40 sec. and the motor sounds much louder and then dies down i called the shop that did the work and they said its a normal sound and if anything was wrong the cel would be on. It didnt do this before i had the work done. When the sound starts it kinda feels like the car has less power but im not sure if its just in my head. any help would be great im thinking about selling the car even tho i dont want to but im just getting tired and hate not have the confidence in the car. Thanks

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2002 A4 1.8t
93oct. apr tuned
debaged
LED tail lights

pretty sure you are now just noticing the secondary air injection (sai). It is pretty loud and the idle sits about 1k rpms. When the car warms up, the idle drops back down to about 800rpms and the noise goes away. Its just a pump to warm up the cat and engine. You dont have a problem

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FACTS > followers
197whp and 292wtq with no chip on stock ic, airbox and catback
AEG crank +Scat rods + 83mm Wiseco pistons = CincyTT Version 2.0T

yeah dude same as above, i had my timing belt and water pump done and mine does the same thing but I didnt notice any difference in performance. Its just the car warming up sometimes cause after a little the rpm's drop back down and you're good to go.

thanks i really appreciate the reply i feel much better.. thanks

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2002 A4 1.8t
93oct. apr tuned
debaged
LED tail lights

Yep, secondary air injection system. It's purpose is to reduce pollution during the first seconds of startup. If it's too loud, perhaps something could be off with it. However, it's a non essential system so I wouldnt' worry too much. Owners have completely removed it before.

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2002 A4 TQMS
REVO 1+ w/Genesis 380s, K&N drop-in, 710N, Neuspeed snub