loud slurping noise

[kams56]

Hello everyone, hoping some one can help me out with a little problem I am having. I have gotten my tanks up and running and cycling but there is a loud slurping noise coming from my drain. I added a sponge thinking that this would help to quiet it but it really hasnt. I think the slurping is a result of a vacuum lke effect the water is being slurped down to quickly. I am not quite sure how to silence it, my thought was to add an air tube to the drain, I'm not sure if this will help anything at all but I figured it was worth a try. I thought about making a silencer but I dont know how to go about making one.

there are two 20 gallon tanks each plumbed into one 5 gallon sump (will replace with something bigger after cycle). it is the top tank that is noisy.
tried to upload some pics but they are to big to upload

Thanks in advance

 

[dannyp02908]

is it a durso style standpipe? If so, the cap should have holes in it. Enough that it can get enough air not to slurp, but few enough to silence the drain.