Welcome to CCUK, Gino. Does it sound like a sewing machine? It could be that the timing chain has slackened with age and rattling as it goes. However, thicker (cold) oil would be expected to make the noise less noticeable than when it has warmed up and thinned a bit. Are you sure it's quieter or are you perhaps noticing it more starting up than when driving? For instance, if you pull over and switch off after a drive, then re-start, is it quiet?
If it does disappear once running warm, it could be piston slap or tappets but better spannerhands can put up an opinion than I can if you'd confirm it's just a cold start occurrence.