Unfortunately the owner lied to u. The.noise at 3000rpm is a big end bearing issue, do not drive the car anymore, the engine will have to be pulled out, sump taken off, crank out and inspected, rods inspected, new bearings put in and te clearances checked, depending how long the knock has been there, hopefully the crank and rods are still ok.
U would have finished it off doing a run in the hills, RB series motors have issue's with oil starvation, and hills are the worst for it if the engine isnt setup with baffling, enlarged oil returns, extra oil in the sump etc.
Sorry to be the bearer of bad news