It is quite common for overhead cam engines to have hydraulic tappets, these are infact small pistons that charge with oil as the engine runs to maintain the correct valve clearance on the cams. If for some reason one of these is sticking slightly (Wear, Manufacturing poor tolerance or production defect) the engine will tap lightly at the top end (Valve gear) until it warms or enough oil pressure is gained to overcome the stiction of the tappet. It is not dangerous or really detrimental to the running of the vehicle, its just an annoyance until the engine has warmed. On older engines I have had limited success using a good flushing oil/carbon deposit remover to cleanse the internals of the engine prior to an oil change. May be worth a go, just follow the instructions on the flushing oil, drain away, change oil filter and add new oil.
Good Luck
Steve
SB
November 2006