Inside each bagpipe drone, you will find special tube-shaped reeds
Bagpipe drones are the tall vertical pipes which rest against a piper’s left shoulder. Inside each drone, you will find special tube-shaped reeds made of plastic or natural cane reed. When blown directly by mouth, these produce an unpleasant sound similar to a duck call. When inserted in the pipes and blown at a steady rate, they produce the low hum heard in the background of bagpipe recordings. You will often see pipers tuning the drones or moving / sliding the different sections of the drones up or down. This tuning gets the drones to hum in unison in order to eliminate a waa waa waa type sound.