Battery drains overnight (twice.) `89 Accord Honda

One morning the car won't start because of a dead battery. I'd jump
started it and went on my way. The next morning, the battery had
drained again. I pop the hood and turn on the ignition and notice the
AC fan is running. Then I charge the battery I connect the battery
terminal to the car. Just right when I connect the battery I hear a
click (a relay click, with igntion off, right below the battery, but fan
not running) that I'd never heard before. The click noise and drain
problem are gone today, but I fear it will come back. What can I
do to prevent my battery from draining again?

Thanks