overflow noise???

I have a bottom drilled 75g, the overflow box is in the back, slightly off center. I recently cleaned the flex, accordian style tube that comes off the bottom of tank, into sump. The overflow PVC is typical, straight piece with a curved piece at the top, basket type piece where intake of water goes, to keep snails out, the air hole is at the top of the curved piece, PVC has a drilled hole about half way down the pipe for syphon. I'm getting a lot of air in with the water going to sump, it's noisy, it's a new noise. When I plug the air hole in top of PVC, letting the level in overflow box go down, then let it fill back up, it's quiet until the water fills back up to its original level, then it's loud again. Any ideas? Nothing is blocking the intake basket and I've not changed anything. Pics are best I can do. Thanks

 

you can attach a short piece of airline tubing to the siphon hole on top then attach a plastic air valve to that, it will help you control the flow, plus cut down on the noise

 

Thanks a bunch, I'll go do that right now!