I would have to disagree with this in part. Once oil pressures becomes sufficient, the loose clearances that were enough to make the rods knock for a second or two after, and not come back, are filled with oil, and the barrier prevent further noise. I would hear this same noise in older V8's that had nothing but a standard nylon coated timing chain. ITs not that the engine is ready to grenade, its jus the clearances are wide enough to take the initial cold start oil a second to fill the gap, so to speak. I had a VW Beetle that did the same thing, and it had a crank/cam gear drive. Most of the time when its a timing chain, it (to me) sounds more like a slosh-type or scraping type of noise until the chain gets tight and lubed. YMMV, of course.I'm not familiar with your consumer protection laws... so that part is up to you... in the USA just about anything purchased used if it is more than a couple years old doesn't have anything in the way of a warranty, essentially buyer beware.. anyway you look at it all the repair people can do for a rod knock assuming it is one would be at minimum replace the offending connecting rod bearing or at maximum replace the crankshaft and all the bearings.. wouldn't necessarily mean a full rebuild... it is worth saying rod knocks don't usually occur at low engine speed or go away after engine startup and can be induced by rapidly revving the engine and letting off the gas pedal... rod knocks tend to occur at higher engine speeds because there is enough play in the offending device to not be able to keep up with the changes in direction of the reciprocating assembly whereas if its just a rattling on startup that goes away after the engine warms up it is almost certainly something to do with either hydraulic lash adjusters or valve lifters ( assuming it has them) or some sort of slop in a hydraulically tensioned timing chain setup.