I keep getting this noise only at 1800-2000 rpm
It doesn't happen aby other time but at that range
What could it be
youtu.be/nSFwggFwaHQ

Ill be watching this thread, hoping someone can shed some light on it. As I have the same noise around the 2000 RPM range and cant for the life of me find what is causing it. Have worked out that the resonance in mine comes from somewhere between the drivers side CV and the sump area however.

I have had SW20s sent to me with bottom end (rod bearing) rattle at around 2k rpm.
 
I haven't listened to your video yet, but it wouldn't surprise me if it was similar.

Sounds similar to my 3sge. I've done a big end bearing. Mine went mid track session.



Not saying yours is the same but sounds suspiciously similar. Question to ask is, has it been getting any worse?

Is there a way to tell if it's a big end bearing rattle before I pull off the sump ?

Not sure if there is. I wasn't sure to begin with. If it is, it will get worse rather quickly the more it runs.

Does it make any rattle on start up?
Before going any further engine wise, check complete exhaust system including mufflers & cats for any loose components.
I have had a lot of cats & muffler internals cause simular sounds.

It's fine on start up , no noises at all.
When I get home I'll check the exhaust system

Unless a rod bearing is completely ferked, most of the time, when the oil is thick (read cold) it will be quiet.  Its only once teh oil thins out that the noise will present itself. 
 
I recently had a 3SGTE at my place with what turned out to be a seized oil relief valve - it managed to basically spin a fresh set of bearings in under 2000kms.  No noise until warm, and only audible at between 2-2.5k RPM.

Hard to say from the video whether this is related, but at certain revs which create a resonant coupling, the edge of the heat shield over the exhaust manifold can start flapping quite vigourously and make quite a strange noise.
 
Easy to check either way.