Go to Halfords and grab some of this stuff in:

Wynn's Hydraulic lifter cleaner. I am no fan at all with any snake oil product, but I will highly recommend this stuff. You shove it in the oil and drive around. Takes a few days to fully work. If the hydraulic tappets are siezed (usually due to lack of servicing and maintenance or running low on oil) then it gets in there and flushes the crud out and the noise goes away. They recommend changing the oil once done, but I just leave it in there. I would drop the existing oil. Put this in with a fresh load, and then leave it for a couple of weeks driving the car regularly. The more you drive it the more it does its thing. Once its done its thing then do a proper oil and oil filter change.
Obviously it does nothing if they are truely knackered or you have other issues, but that would be my first cheap easy port of call.
The smoke could be anything. What colour is it?