My Punto Eleganza has a knocking sound under the bonnet for a couple of minutes on cold startup especially in this cold weather. I turned off the ignition after it began knocking this morning and restarted - instantly the knocking went. The car has only recently had its three year service so it doesnt need an oil change, and Im not sure about adding any metal treatment to the oil because it seems it causes greater wear in the long term to the engine.

How can I reset the ECU to see if it makes a difference?

I have the same problem on my 2001 Go! for about a year. I reset the ECU but it didn't work so I've just left it for now.
It's going in for it's 36k service soon so I'm gonna see if that makes any difference.

I wouldn't put any addatives in because these cars should run fine without them.

I wouldn't worry about it, if you can put up with the noise then just leave it, it hasn't done mine any damage... yet.

if its just been serviced check that they didnt forget to re fill it, also make sure its nice an clean some bodge garages dont change it properly, they only let it drain for a sec and leave half the crap in there