Timing chain tensioner loosening
A common problem some Corsa owners report is a tapping noise from the engine area, when you listen it can sound like something is rattling inside the engine. The cause of this is generally a loose timing chain tensioner. The tensioner, as the name suggests, keeps the correct tension on the timing chain, when this part starts to fail the chain can become loose causing the rattling sound.
What to do:
Quite often the problem is mild and the tensioner is fine to carry on as is, you'll just have to accept that it's noisy. It should be noted that if it eventually fails then the damage to the engine would be catastophic.
Unfortunately it's not a straight-forward job to replace it and involves a fair few hours of mechanic labour. Try to avoid cars with very noisy rattling engines, if yours is it's best to see what your mechanic thinks.