I haven't found out the reason yet either... I took my front off the ground and I don't hear it at all, I'm not hearing it in park, neutral or reverse...so I'm not sure.... I doubt it's cause of your valvue adjustment...

I tried to start the car on a cold day and immediately put it drive and had the e-brake up ( I have an auto of course ) cause i think you can fadely hear it before the car warms up...it sounds like it's coming from the driver side engine bay like down past the cam sprocket close to the alt.