Just gabbed the fan and checked for movement. Fan/pully has noticeable play. Looks like I found where to start taking things apart at... Thanks for the tip!

 

Check your balancer, they are known to flog out the keyway and worse case fall off.
I had one slip off the crank 200m from home, walked home got a spare crank bolt and i was off again in 30mins.

 

I had the same issue with engine knock and think it was temperature related. I replaced the water pump with a HD version, fan clutch, radiator, rover thermostat, reservoir cap, electric fans, and changed to G5 coolant making sure to bleed the air out of the system. I also did an oil system flush and started using 20W -50 and have no cooling issues or engine knock.