My home theatre speakers have been making a hissing sound when my avr receiver is turned on.

My home theatre speakers have been making a hissing sound when my avr receiver is turned on.
Because the power is free flowing.

Because the power is free-flowing.

Is there a signal on the audio input you've selected? (i.e. CD playing, etc.)

Hissing can be caused by EMI noise coming off the ac power cord and coupling onto the amplifier circuits. Your speaker wires may be running parallel with power cord(s).

Try plugging the receiver into its own ac outlet. Try rerouting speaker wires away from power cords and at 90 degrees if they have to cross.