Sounds eerily similar to a rod? Does the knock happen right when you start the car? Does it only happen under load (ie. when say in gear and bringing up rpms)? Have you redlined the car hard recently?
Does it sound deep into the motor? Cause if its a rod your days are #ed and it would be best to diagnose it before it completely goes? If not you could have issues with the block as well :dunno Find a motorhead in your area ask him to listen and see what he thinks?
Clutches, from my experience, typically don't knock and you would most likely have a foul smell associated with the clutch?
I threw a rod in cylinder 1 at 72K miles ? And the car was flawlessly maintained with synthetic oil and everything done top of the line? It just depends on the car? When the motor was being rebuilt it got beefed up though

stroked and forged, and a new clutch, water pump, motor mounts, and other needed b.s. So basically I have a new car from behind the radiator to the tranny?
Oh and I replaced an alternator and battery at about 58K if I remember right, pretty basic and common though.