Rena - quiet as they come; Ive got the 400 its nearly silent.
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=3724&N=2004+113800
I also have a whisper 150, it is also quite, but not as quite as the Rena.
http://www.tetra-fish.com/catalog/product.aspx?id=69
I like the pumps with larger bases to dissipate the vibrating ...
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=3724&N=2004+113800
I also have a whisper 150, it is also quite, but not as quite as the Rena.
http://www.tetra-fish.com/catalog/product.aspx?id=69
I like the pumps with larger bases to dissipate the vibrating ...