If the sound goes away with the clutch depressed, that signifies...........?
After I set my valves I heard a noise somewhat similar to yours.It would go away when I shifted.
Ended up being exhaust leaks at the flange and manifold.
(may not be what you have just something to look at)I had a machinist deck the manifold, replaced the doughnut gasket at exhaust flange and no more noise.
I'm sure you will get a good idea on whats going on when you look under your cover and take your valve readings.
could be a number of things, approach it one item at a time and you'll nail it down.
