I bought these ones at walmart http://www.walmart.com/ip/Aqua-Cultu...ndingMethod=rr and they've been doing a great job for me :) Mine is about 3" long. There are also small ones the size of the tip of your pinky finger that do a good job. I like the longer ones from walmart because the plastic casing keeps the stone weighted down and they're very inexpensive.
i debated on buying a marina elite 50 or a tetra whisper 10. they both sold for around the same price at my local pet store.
i had old style tetra whisper pumps in the past. they used to be square or rectangle brown boxes. they would fail and sometimes work but just get louder and louder. the new ones look totally different on the outside but i dont know how different they are on the inside.
so i decided to try the marina. so far i am very happy. it sits on the carpeted floor about 4 feet below a 15 gal tank and runs a foam filter. my gf and i cant hear it at all.
if your whisper pump fails you could try the marina.
My Marina was very noisy. The cheapy that came with the Tetra 3i is quieter but weak. My Tetra Whisper is just as quiet but stronger.
It's still too loud. That and the pH raising tendency of sponge filters is why Im getting rid of all of mine.
I find that if the pump is on the furniture or wood floor the noise is quite noticeable but on the rug not a peep. As for breaking the diaphram in my tetra is rip around the out side but it sill flows plenty of air to run my bubble filter.