A friend of mine has a 98 Ford Explorer XLT that has an intermittent
problem - a "hum" in the air intake at times.

When you lift the hood (when it's making the noise) you can hear the
hum and putting your hand on the air intake (the box immediately above
the air filter) you can feel a vibration that's not there when the hum
is not there.  Taking the air intake off the filter makes the noise
much louder.

What I've noticed is that giving it some gas will make it stop, but
when it is at idle it will return.  This can mean either when it's in
gear at a stoplight or in park sitting in the driveway.  We've not
been able to find any way of reproducing the sound on demand, but it
happens frequently enough to be annoying & worrisome.  It doesn't seem
to have any set of circumstances that makes it occur, the most recent
one was at the end of a 3 hour drive.  I've also heard it myself on a
cold engine.

A couple questions -

1.  Is this noise harmful?

2.  What the h-e-double-toothpicks is causing it & how can I make it
stop?  Someone I know who owned a 2000 Ranger also experienced this
noise, and other than identifying it originating from the air intake,
couldn't get it to stop or do anything.  I have a 2003 Ranger that
does NOT make this noise (knock on wood).

Any help/advice/suggestions appreciated.

SC