[Montync]

I would check both, flex to crank and flex to torque, You might be able to see the cracks, fractures. If not then run it until it knocks and get underneath and see if its hitting the bell-housing (wobbling) also look for rubbing. As far as it not doing it all the time my guess would be that when it heats the metal, it expands and somehow causes it to hit the bell housing. Almost like an exhaust leak, when cold no noise, when hot noise......Oh check for any exhaust leaks to near the collector