what's your valve clearances like?

I'd give the head a once over, check for stuck/sticky valves. clearances. get the intake off and check to see if they're all closing fully.

also, when you say "more of a clack" would make me think a blowing exhaust close to the head, check manifold/gaskets etc.

what colour were the plugs when you took them out? if it's running slightly lean you could have burnt an exhaust manifold gasket out