Gurgling Sound in Sump

my tank creates a gurgling sound here and there and it is annoying as well when sleeping being the tank is in my bedroom. I have dorso fittings on both drain pipes and still making the sound. the guy that plumbed it who i bough it from went from a 1 1/2" drain pipe to union, then from there into 1". i was thinking to re plumb and do 1 1/2" all the way down. any suggestions??

 

I woudl redo the plumbing and replace the 1". If it's still gurgling after that drill a small hole in the caps on the dursos.

It's more effective it drill a set of small holes to tune the overflow than making one hole larger each time until the overflow is getting a good mix of air.