The suggestions above are good, but one thing you don't want is the valves adjusted "Slightly Tight." This can burn your valves. Better a hair loose than tight.
But that won't be the cause of you clanking noise. Assuming your hearing is good enough to detect an engine noise as opposed to the front fender, my first guess would be the small end of the rod. Or as you said, piston slap. As you try to detect the spot, if it's coming from the bottom end, it's probably a main bearing. If you ultimately have to tear down the top end, pull the timing cover off and check the seal at the end of the crank. Very important to oil pressure and engine life.
Bare