As a rule of thumb:
Black Smoke - Unburnt diesel - Check the boost pipes for any splits. If there is a significantly higher ratio of fuel : air because air is escaping and not going into the cylinders, you will get plenty of this stuff. Alot of diesels give out a cough of black smoke on startup and under heavy acceleration, but it shouldn't really do it at idle speed.
Blue/Gray smoke - Combusted Oil - First place to look would be the breather pipes to see if oil is being injested through the intake manifold. A common cause for this on an old engine is blow-by pressure in the crankcase or rocker case caused by worn piston rings or scored cylinders. In bad cases the engine will use this as fuel and self rev on it, meaning that you'd have to stall the engine or it'd drink the oil until the sump was dry and seize the engine. Other causes could be blown seals on the turbocharger, worn piston rings or cylidner walls scored, or leaky valve stem seals.
White smoke - Water/Vapor in cylinders - More often than not a blown head gasket.
How much suction is created in the rocker cover? You'll always get a little bit of pressure in the rocker cover due to the cams spinning round.
Are you sure the smoke is white and not the gray colour I've mentioned?
Is the air filter, and intake air pipework clear and free? If the source of air is blocked, the engine will try and take air from the next available source, which is likely to be through the breathers connected to the inlet manifold, which could be a cause of your suction in the rocker cover.
Check your breather pipes for oil. And check that the intake pipes and air filter are clean and free... that would be my first move.
