2009 TE 250 Crankcase Pressure

I took my newly rebuilt 250 out for a run today. There was a very slight noise on light throttle (the chain might be a little slack), but sounded generally good. I was having a look round after to check for any leaks and noticed that the oil was a little bubbly in the sight glass when running. I took out the fill plug and there's some pressure in the crankcase. Take the breather tube off the head and there's a steady blow. Is this normal? Post rebuild paranoia?

OK, so a quick clean up of the chain (I'd throughly cleaned it and then oiled, so there was a bit of fling) and check the tension. Bang on spec. Messing with the chain and I noticed a side to side rattle, just what I was hearing on light throttle. Rear guide was loose, I missed it on my final round of fastener checks. Doh!

Breather / crankcase pressure question still open for info!.