Very common, pretty much every 4 stroke engine on earth does it, even more so in turbo charged diesel engines where the compression in cylinders is considerably higher than that in petrols. In most cases it is simply compressed fuel & air escaping past the piston rings or through the piston ring gaps. You only really need to worry about it when crank case pressure becomes excessive. A simple check is to remove the breather pipe and place your thumb over the end for ten seconds. If it sounds like a traction engine letting off steam and it launches oil at you as you remove your thumb then further investigation is required (e.g. worn bores / rings / cracked pistons / head gasket blown between cylinder and oil way etc). If it just hisses and releases a small amount of pressure then there's nothing to worry about.
-Tom