they are noisy my friend, but notheless still very fantastic device. basically, the poor mans jet engine. really simple. those germans used them for the flying bomb in ww2

anyway, i am sure u can reduce the noise.
how?..

add a shroud nozzle encasing the tail pipe! must extend bit further than tail pipe!
function is 2fold
1. high velocity of exaust stream will interact & pull in more air creating an overall higher mass airflow. greater thrust.
2. interaction with slower ambient air inside the secondary nozzle will "muffle the higher pitched notes! much like what happens in a turbofan (compare it to a turbojet)

oh yeah, interested in building one? found a really cool site with free plans http://home3.inet.tele.dk/kennethm/index.htm
so go check it out man