1999 Vauxhall Tigra oil light and tappet rattle

hi there I have recently brought a 1999 1.4 16v tigra just done a part service oil/filters /plugs ect ect.its been running fine but for some reason the oil light has been showing and this is promptly followed by what seem to be the top end of the engine rattleing presume tappets but no smoke. However if i let the car cool down it all stops and the car runs normally, it has only happened twice so far but I do not want to do any excess damage to the engine. Having a fare amount of knowledge I am presuming it is some thing to do with the oil pump/pickup pipe/oil pressure. Would like some advice before stripping engine and re-building and spending hundreds. All comments appreciated. Regards Keith Hill