If you can't find a check valve you like, but are worried to not have one, then a quick fix would be to make sure your pump is higher than your tank. If you have any way of doing that, then a check valve wouldn't be necesarry since gravity would prevent it from siphoning.
As far as not hearing an air pump at all, I have never found one that was silent and that's what it seems like you want. You may want to look into putting a silencer on your pump as for the most part it's the noise inside of the tubing itself that causes the noise. I have found DIY instructions to make one on a member's website, but I can't remember who. But I remember it was a guy who said he bought his house specifically because the laundry room wouldn't finish and he would be able to complete his custom setup which included switch controlled
PWC's. Hopefully he will see this post, or someone will know who I'm taking about and can post his site.
I know the very basic instructions involved just running your air tube through an old water bottle that had a hole drilled in the top and bottom and was stuffed with cotton. But his site had more details and pictures.
-brent
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