It started happening a couple weeks ago, I notice it most at idle because I can't hear it over my exhaust when driving.

It increases with rpm.

Pressing the clutch down has no effect on the sound, it is constant.

http://www.youtube.com/watch?v=m3aea7_qi_U

 

VCT SENSOR. My first guess.

Does it Crank Hard. As in the engine seems to fight itself to turn over?

 

Get a long screw driver or some kind of steel rod & use it to isolate the noise. One end on the engine the other on your ear. Hunt down the noise. Hard to tell, but could be a lifter is collapsed & you are hearing the rocker making noise. Be careful, you can suck a valve if that rocker gets jammed between the cam lobe & valve spring.