Burping overflow driving me nuts

[Jason McK]

I have a 75G AGA with the "twin overflow" plus the derso styled AGA Mod. I have just rebuilt my sump to accommodate a new skimmer and hard plumbed the overflow tube with PVC. The overflow goes strait to the bottom of the sump and there is one 45 elbow at the end. Now I am able to increase the flow through the overflow but the more water running through the louder and more frequent the "Burping" sound becomes.
I have tried adding a flex airline tube down the overflow pipe but this does not seam to help unless I need a bigger tube or if I need to find the sweat spot.

Any help would be great

Thanks
Jason