It sounds like you may be hearing the infamous MOOSE.
You might be hearing some noise from the drive belt idler pulley, belt tensioner pulley, or even the alternator, but those noises don't usually come and go. Also these items are on the right side of the engine.
If it is the moose, it will seem to come from the air filter box or from the idle air control. These items are on the left side as well. Sometimes cleaning the idle air control will eliminate or at least decrease the noise. The moose seems to come out when there is weather changes, either hot or cold, but more often in cold weather.
