Finished my engine swap! Wooo. Now though, when the engine is cranking it sounds fine, but the moment it catches it starts this noise. It sounds like the fan hitting the cowling, but it's not. Please don't tell me it's a bearing gone.
It changes with the idle too.
Please help, right now I feel like crawling into a corner and curling up to die.
1989 2.5-16v.

I had a noise like that and it did turn out to be the fan hitting the radiator cooling shroud. (It wasn't obvious from a casual inspection).

Well, here's the vid:

http://video.tinypic.com/player.php?v=3yfcjrs

It's 3mb, so not huge. The problem is you can't really get a feel for just how LOUD this sound is!

I've tried running it without the drive belt - the noise is still there.
I checked the timing - the TDC marks on the cams are 180 degrees to each other.
Oil is circulating fine - nothing is dry.
I really don't know where else to look.

Since the sound has a low frequency it looks like a noisy valve. If your engine has hydraulic adjusters one may have failed. If not check the valve play.

This is the latest vid. It's with the rocker cover off and the sparks out so it's just cranking. I was wrong earlier, you CAN hear it without it catching, just not with the valve cover on.

http://video.tinypic.com/player.php?v=29ofyf5

The cam video seems to prove Hr. Prujitt right. It sounded like it was noisiest toward the front of the engine as the camera moved.

__________________
Prost!

Rob - no lifters on these unfortunately. It just goes cam -> valve with a shimmed bucket in between.
What I have had suggested, which I will definitely check out, is perhaps a broken valve spring.
Thanks for the replies. Any further advice/thoughts are most welcome.