The Apple II power-supply is fairly "intelligent". If it detects a no-load situation (that is, nothing is taking the electricity), it will short-circuit itself indefinitely. This will come out as a rapid clicking noise, which is quiet, but can still be heard. Likewise, if there's ever a short circuit, the power supply will shut down for a moment, then attempt to restart. If the short-circuit condition is never removed, this cycle will happen indefinitely, producing the same rapid clicking sound as before.