sump noise problem

Hi,

I've made new 700 l aga aquarium with 225 l sump. They are connected with
two overflow boxes, both with durso standpipe made of 1.25'' pipes and
fittings. The durso works fine, there is no gurgling noise. Noise comes from
the outputs of the pipes that are submerged in the sump. I run the sistem
with german ATK pump that has 5000 l/h flow at 0 feet.
I've tried several things to make it quieter but am still hearing the noise
of large bubbles coming out of the pipes and emerging in sump.
Since I'm only testing the sistem now I'm using fresh water. Maybe this is
the reason why large bubbles come out of my pipes.

Can someone give me some usefoul tips how to reduce the noise.

Tnx

Mislav