I've made a brief list of other things that could possibly cause a noise like this, some thing are doubtful but anyway here's some of it, let me know if you think any of these are common/likely or whatever.

• Injectors tapping due to being clogged
• Clutch buzz from the thrust bearing (although this is unlikely as the car still makes the noise when the clutch is in)
• Metal header gasket buzzing against the head (also unlikely as I've removed the header and re-fitted it with sealant on the gasket)
• Exhaust leak from the header area (I can't see any cracks or holes though)
• Crack in the header mount (someone had a similar problem with their prelude and said this was the issue, haven't had a REAL thorough look at mine but it looks okay)

If anyone comes up with ANY idea's even if they're super unlikely please post!