Fear not! All maybe ok with your engine. I've heard this noise before and it was the alternator clutch pulley. To confirm: remove the aux belt and run the engine. If the noise has gone your engine has life in it yet.
I think dgmayhead is right. It doesn't sound very big-end to me, it's too fast for normal tickover engine speed, and you normally first hear big ends on the overrun. By the time they become loud at tickover there's no oil pressure and the engine is wrecked.
Sounds like a seizing follower or something on the top end. Can't imagine you'd hear the alternator from the camera position. If it's the same with the belt off, I'd take rocker cover off and have a look in case a bearing cap is loose or something, especially if it's had timing belt recently as you have to remove caps on both cams to lock it.