If you're running a 2.2L it could be hydraulic lifters sticking and causing your noise or it might even be your timing belt tensioner. I'd check out those areas to rule out two of the most common causes of the noise and identify why it's inconsistent. Also, as was mentioned, if you've got ~100K miles on your car, it's probably time to do a valve readjustment.
Could be a batch of bad gas or octane too low during the hot summer season as well. This is the time of year when the humidity and the heat conspire to stress your engine, particularly the heat, which can cause an ECU ignition retard and pingo goes the engineo.
Just a couple of thoughts. Keep us appraised of your isolation progress.