Could be something expensive to fix like the bottom end being gone, could be the oil pump on its way out, but that will more than likely be followed by the bottom end anyway.....

Could be blocked oil ways in the head if it hasn't been maintained well.

Having just re-read the post though it seems i got the wrong end of the stick, i thought the noise started when u revved the engine and that it was silent at idle.

You say you blew the oil cap off? And now its missing.......sounds expensive to me.

As per usual a compression test would never go a miss