The fuel in the combustion chamber is not burning completely and seeping down the cylinder wall(s) into the crankcase. The spark plugs may be fouled or possibly one (or more) is not firing. You might also want to check for a clogged air filter.
This can result from heater core blockage. If it is partialy blocked, you may notice a little heat with the fan on low, but it goes cool when the fan speed is increased. Other signs of blockage may...
The clicking may be the belt tensioner clicking. I know that mine is causeing a clicking noise. If you get a friend to help you, you will be able to see it happen if that is the case.
Possible causes: (1)insufficient coolant in the system (2)water pump drivebelt defective or not adjusted properly (3)radiator core blocked or radiator grille dirty and restricted (4)thermostat faulty...