Now I am stating the obvious, you are topped up with engine oil aren't you?
Where does the knock seem to come from, low down the engine, which would point to main or Big end shells. Higher up could be either of the camshafts or a dropped hydraulic tappet although they tend to be more of a tap tap sound. Cam chain & tensioner tend to rattle a lot when worn on the 1.0i Corsa.
Don't be driving it around until it goes bang and throws a leg out of the bed, otherwise its a new short motor for you
It's possible to get the crankshaft machined and fit new oversized shells.
Best get it checked ASAP.
Also wondering why the noise only happens after 5 mins, ie when its warmed up. Most knocks due to worn bearings are most evident when cold and metal has not expanded and oil is cool.
Just check the catalyst mounting bracket is not broken as they tend to do on 1.0i Corsa's as this can make a right old noise
cheers
CJ
ex-Vauxhall Service