The N75 valve is bad. The only reason I knew to suspect the N75 was that since I've owned the car, every time I close the hood after it's recently been run, the firewall would hum loudly for a few seconds. I thought it was a defective alarm horn or something, but a few weeks ago when I was pulling my EGR valve, I whacked the intake manifold with a dead blow hammer to break the valve free and the car made the same humming. I put my hand on the N75 and could feel it vibrating, so I pulled it, tested it, and it was definitely leaking vacuum. I went ahead and replaced all the lines and the vacuum ball as well. The best part is, the replacement part was free - it's exactly the same part as the valve that controls EGR on the ALH TDI cars. Now, the car flies like a bat out of Hell again!
