Sometimes, the plastic oiler tubes that lube the cam lobes break, causing noise. A lifter that needs to be shimmed can also cause this.

Though, if the noise come along all of a sudden, I would pull the valve cover and replace the oiler tubes first. It takes around 1/2 an hour. No special tools needed, other than a 10mm socket for the valve cover bolts, and screwdriver for the breather hose on the valve cover.

Another thought, I had the same thing happen on a 450SL once, and my dad joked that a sparkplug might have be coming loose. I thought he was crazy, until the dang thing spit the plug out a few days later at 70mph. It also ran well, idled great and all that.