2005 Opel Corsa Engine noise

I have a problem with my corsa GSI 1.8 in the morning it makes a tapping noise in the engine and when is hot is quite.i took it to opel and they told me that it is normal which a dissagree with because when i bought this vehicle it has no problem only after 2 years the problem started.what can it be and how to correct it.my common sense tells me it is hydraulic lifters they worn and the gap in between the lifters and camshaft is big.can i overall the engine with new lifters?please advise.