I just noticed a very high pitched squeal from the rear passengers side of my 2004 A4 1.8T engine(sounds like a vacuum leak) I tried to remove the oil filler cap on my engine, but couldn't(negative pressure) I shut the engine off and restarted and the noise had gone away and the cap could be removed. Reved the engine and the noise started again and again the negative pressure in my crankcase. Anybody have any ideas. Hoping for an simple fix, but have had the car for awhile and realize that there are not many of them!! Before this I noticed that when it warmed up and idles down to 800 it goes just below that and is a little rough. Lots of power and the turbo seems to be working fine.

 

hmm i'd vote bad DV valve

 

Is that the valve that sits on top/rear of the air intake box. Just replace the air filter and had to unhook it to get the filter out....didnt unhook the electrical just the vacuum hose. Thnks.

 

No, the dv sits pretty much in front of the turbo.
I believe your problem is to do with the pcv system.
I saw something about this recently.

 

ok, i found it.... here is the link

VWVortex.com - Rough Idle, Tough to remove Oil Cap, Codes

 

Yeah I think it is probably the Crankcase Bleeder Valve Assembly by what it said on the link you gave me. Now just have to find the part here in Canada....will probably have to go to the dealer as none of the usual sources seem to have it.

 

Thanks for the posts guys....really appreciate it....thought it was the turbo originally when I first heard the noise so this is a bonus!!!!

 

This was the problem. You can get at everything by removing one screw on the coolant tank and moving it to the side. When I took it apart the hose that connects to the oil filter housing crumbled and the hose that connects from the t valve to the valve cover broke as well(both were very brittle. The t Valve is what had malfunctioned and was stuck in the open position. Small pieces fell down from the disentigrated hose into the filter housing. Rigged up my shop vac with some R/C gas line hose and sucked all the debris out of the housing. Put everything back together after getting parts from the dealer(two hoses and a t valve $240 dollars), reset the codes and everything is back running like it should. Thanks again guys, this would have been an easy 6-700 dollar bill if I would have taken it in to the dealership.