Ok so I've been searching thru trying find my exact issue but have only come up with similar ones. I recently purchased a 02 a4 1.8t quattro.

So here's my issue i have a humming/buzzing sound coming from underneath the car. From what i read it's typically a fuel pump, but here's why i think it's not.. The noise doesn't start until I've been driving for about an hour, it fluctuates higher and lower pitches with idle and acceleration. At idle and laying under the car it seems to be coming from the resonators, when inside car driving it gets loud during acceleration and easily heard but at idle you can barely hear unless you rev the engine.

Can anyone help me please, I've already done timing belt water pump and thermostat within first month of ownership I'm hoping for easy fix.

 

if its coming from the back under the passenger seat- the sound is the fuel pump.

it's not necessarily the pump going bad so much as straining to work through a clogged filter. change that and see if the noise goes away.

 

Would that only be heard from inside? Cause my noise is loud outside the car as well..it's almost a vibrating rattling sound....

 

And it's also louder on drivers side oppose to passengers

 

And the sound fluctuates in diff rpm/gear scenarios 1st gear take off is its loudest.

 

https://www.youtube.com/watch?v=j7o80WYtTv8&feature=youtube_gdata_player

This quick video i took you can hear sound when i accelerate. Will try do better later

 

If you're talking about the faint *thumpthumpthumpthump* sound that sounds like a bushing on the lower control arms, or one of your front wheel bearings.

 

Actually after taking it to my warranty mechanic they were able to determine the source of the noise was coming from my resonator.apparently something has gone defective within it and isn't covered by warranty. My solution until i get my cat back system I'll just remove and straight pipe it.

 

the straight pipe with no muffler? you will get annoyed with the sound...

 

x2! Just get a used resonater off ebay for something. Or check vortex for a part out!