If 1.8 gal are discharged in 15 seconds,
then 231*1.8 cu in of air must be expelled in 15 seconds,
through a 3" pipe with a cross sectional area of 7.0786
a rate of 14117.64 in/hr
or 14117.64/(12*5280) .2228 mph
Speed of sound: 767 mph
or 0.00029 of the speed of sound
or 3442 times the speed of air flow