Hi freddy ,
it depends on how much you put over them , and where there sited , and how the water re-enters the pond .
they are not silent like 99% of koi filters , so a carefully worked out bakki and its position , is essential
i'm not saying there really noisy , on an average sized pond 2-3000 gallons , with the correct pond return , it could virtually be silent , plus a loose fitting cover made from poly carb , would cut it down further and insulate it slightly .
if we was to use mine as an example of noise :
my shower is 3 tear 12ft long and has 30,000 gallons per hour going over it , if i remove the return shoots and covers , i can hear it 140-150ft up the garden !!! and siting next to the pond is quite unpleasant as it is very loud !!
once i put the covers on and the shoots back in place , you only start to hear it 20ft away from the pond , and when sitting by the pond , it sounds like a small water fall
its all about returns and covers , once this is sorted , and is at a level where no one would complain , you have the best filtration system in the world !!!
steve
kois and ponds 
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