Okay any other too common areas for this to come from is either the intake manifold Being a vacuum Leak . All often the PCv valve may be faulty causing this problem , and sometimes the front transmission seal will squeal and making noise
Start the engine so that makes the noise and remove the oil filler cap tell me if the noise goes away
Yes this is most likely going to be either like I said a faulty PC the valve or a leaky intake manifold gasket . If you had a smoke machine would be able to track it down a little better .
If you spray the area where the noise is with carburetor cleaner does the engine speed or idle quality change at all ?
get a 3 foot piece of hose , place it near your ear and use it like a stethescope , slide it down under and try to pinpoint the origin of the noise
If you could take a picture of it and send it, that would help tremendously