One differences between L77 and L76 is that the L77 does not have the DOD lifters like in the L76 AFM cars. The DOD lifters are designed to blead down when AFM mode is activated. They can made a slight chatter when pumping back up but normally for only a 2 ro 3 seconds. The L76 has 4 DOD lifters and 4 normal.
Not realy hearing the nosie your suffering from and for how long it goes for makes it hard to pin point. Maybe upload a sound or video file of the noise. Is it worse when the engine is hot or cold (such as first startup). I have had 3 engines relpaced in my ssv to rectify startup rattle (sounded like a captiva) till the one I have now (later model L98) seems ok. However my issues might not be the same as yours.
From what I have been advised, these engines should not make any abnomal ratteling, clucking, deisel engine type noises at all. I would persist with getting it looked at quickly an don't take the "thats normal" explanation. We have many of theses cars in our fleet and I tested 7 and none made the noise mine did.
Good luck
P.S. Changing your oil probably wont fix the issue (it may mask it) so save your money - I spent lots of money doing the same thing