These are extremely reliable engines, especially the series one L27. Most last several hundred thousand miles with normal maintenance before needing any overhaul service, and even then, valves aren't an issue.
A knock? Maybe a tick? Possibly a clicking? Is there any way you could get a sound clip of this knock? Or at least isolate the sound to one general area? These engines are not completely silent either, they have their own sounds, small ticking noises and such. Is this a new noise? If you place your hand on the intake plenum while the car is running, can you FEEL the noise?