Check you engine mounts, Gearbox mounts and exhorst mounts and alt brackets etc. Test and tighten all the nuts and bolts you can find and reach. Every time I take my race car out I do this. lots of things in a datto will rattle and clunck and your always sure its the engine. Try and eliminate all other possabilities. It will be easier than taking the flywheel off and if its not the problem at least you'll know everything else is nice and tight.

Quinn