Firstly i have a audi a4 2.8 12v quattro, today idownshifted and starting pulling to pass some traffic out of nowhere i hear a something like a turbo spooling up whn i was at about 3600 rpm i kept pulling and shifted at 5000, when i let off the gas the spooling stoped and was replaced but a light shhh sound, this is not a turbo car nor does it have any gay mods to make it sound like one, to make sure this wasent a random thing i drve the car home, shifting under 3000, pulled into the garage and reved the engine, at the lower revs it sounded like it should, a bit loud due to a muffler leak and cats that have the honeycomb completly burned and now they just rattle, as i reved higher into the band of 3k to 5500 it did infact sound like a turbo spooling and a bov. very soft but this is the best i can describe the sound. what could it be. is it the cats, i know they are burned out but they still rattle and i get backfire when i engine break so it sound kinda like a lamborghini, it hasent thrown a code since the day i bought it, i know there was code that i cleared when i bought it but since the day i cleared it the CEL has never come on, once or twice the engine seemed sluggish and i pulled a code for a EGR valve but it still didnt set off the CEL, this is really annoying me, do i have to be worried, i will be replacing the cars in the spring but is there any harm in running the way it does, avant80@2.6 i know you can provide and answer.

 

Sounds like a intake leak, or maybe the throw out bearing.

does the noise change when you push in the clutch?
I do know when the syncroes go out, they make a kind of turbo noise, but you have to be moving (car in gear accelerating) to hear it.

 

i doubt its a intake leak, it was definetly coming from under car somwhere between the cats and the resonator, it heard the noise when accelerating and the shhhh when i let off the gas(without clutching in) same thing with clutching in and then just reving the engine in the garage without moving, i really hope its not the synchro, also this may help it made th enoise agian on my way to school but i turned of the climate control and radio and it kinda sounded and felt like something was clogging the exhaust kinda like backpressure was building up, but i could still here a fairly quite yet clearly audiable sssssssssssssss-shhhhhhhhhh.

 

Check for an exhaust leak. The area around it will be black soot.

Other then that I have no clue.

 

EGR would throw a code but the cats sound like the problem also any leak in the exhaust would cause this sound

 

you could fit a test pipe which is cheaper than a cat

 

exaclty what does a test pipe do, i was planning on getting just some universal cats off ebay and have a local shop weld them on to keep me within emmisions limits. as long as nothings gonna mess up on my cat ill spring im good, i just want to make sure this noise isnt hinting that i got silicon build up or something.

 

it replaces the cats and gives better performance and better fuel consumption but depending on where you are in the world the emissions could be a problem, its just a make up pipe that takes the place of the cat