Noisy return pipes (bubbling) -- overflow is quiet

[SteveW]

I have a 155G bowfront, with 2 built in overflows. It has 2 All Glass Overflow kits (one for each overflow), which includes a 3/4" return (connected to my Mag1800 pump) and a 1" drain. Sump is a cheap 25G Rubbermaid, unpartitioned. All tubes are connected using a straight barbed insert (no elbows on the drains).

The system is plumbed using PVC hose (not rigid PVC). The drain hoses are 1" braided (reinforced with fabric mesh inside) PVC hose, running freely into the sump.

The overflows themselves are quiet: no gurgling, no flushing sounds, constant water level within.

The problem is in the sump. As a result of air mixing with the drain water, the drain hoses make a ton of bubbling noise: large bubbles come out at the ends. I can see the hoses themselves are not "full" of water - I can see the water level coursing within them as it rushes back to the sump. It sounds like large pot of boiling water. I tried a smaller pump (MagDrive 7) but it had the same problem (although a bit less due to less flow).

I tried submerging the hose ends in the water a bit -- tons of bubbles no matter the depth, and there is too much splashing sound if the hoses are held above the water level.

What can I do to fix this? Would using elbows help? Should I change the hose to rigid PVC (and why would it make a difference)?

Thanks...
- Steve