I ran around the engine with a screwdriver to my ear testing sounds last night. The sound is coming most strongly from around the intake manifold (on the passenger side of the engine really- I could only get the screwdriver on the manifold to test it) rather than the front of the engine.

Even the slightest pressure on the clutch will stop the noise- it doesn't have to be fully depressed. The noise gets louder as the engine revs up.

(I checked the pulleys with the screwdriver and couldn't find the sound).