A friend s nova has a slight prob

2L 16V xe engine.
Basically its standard but been rebuilt (rebore) new bearings(bottom end) ect ect

For the first few miles there wus a bit of smoke but now its cleared up. But what ive noticed is.

A slight like rattle/metal noise. now last night i happend to pull is dipstick and its running right on the bottom line of the minimum mark. So ive sent him for oil today.
more noticeable on idleing that anything.When revved it dosent get any worse. infact becomes less noticeable.

Could it just be starved of oil . (not good after rebuild) or should i be looking for more serious problems ?

To be honest it sounded like an external noise. but im yet to investigate it fully due to me working long hours.
ive sufferd piston rattle myself but this dosent sound like that. plus there wus no rattles after rebuild. engines only done 600 mile since .

Anyone shed any light on this

Be glad when mines run in meself!!!!!