I noticed today that the gap in one of the front wheel wells (gap
between the top of the tire and body) is way wider than that of the
three other tires;  I can fit my fist inside.  The other gaps are
relatively narrow.  (98 accord sedan).

That's probably explains why the car is making a harmonic noise at high
speeds.

The last thing I had done was an oil change and tire rotation, outside
the dealership.

Anybody have a clue as to what is out of whack?

TIA.