From our good friends at ECS Tuning:

https://blog.ecstuning.com/2017/09/1...e-replacement/

 

Hey guys I wanted to share.
My coolant flange had an invisible crack in it.
The thing is 10 dollars.
​​​​​Don't reuse the old one.

 

I replaced mine 2003 A4B6 2.0 its a easy job.
I had to replace the ( electric ) thermostat ( inc housing # 06B 121 111D ) and that rear flange at the same time. ( # 06B 121 133 )
It cracks its support ring inside & the 'O' ring moves & leaks ( which might be why its not leaking under pressure, as it will get pushed outwards & reseal again )

i also had to replace a coolant hose part ( # 06B 121 058 ) because one end had went really soft & its less than 1/2 the cost of the coolant it would loose if it burst.
So expect that if one of these parts need replacing, they all will. If i had the $$$ i would replace every hose as a kit.
You can buy that rear flange as a Aluminium version too. Ebay it.

I also replaced the Crankcase ventilation hose (# 06B 103 211 H ) $5 ebay China ( just cut its neck off to remove it easyier ) and the top housing it connects to ($30 VAG ) i used rubber tubing to connect that to the intake ducting ( too many $$ to buy a new one ) and a 1 way valve close to that.
Check all this stuff at the same time.

A couple of months later, two door mechanisms broke, easy fixes, cheap from China.
What I mean is, is that all of the same parts will break at near to the same time, so double check everything & buy the bits before they need replacing. I sound insulate the doors while i fix the window regulators ( winders mechanism ) & look at replacing the speakers too.

 

Is it on the passenger side leaking on the exhaust then it is likely coolant flange or hoses connected to flange.