you wrote:
When I set up my tank I used flexible pipe from the overflows to the sump. My tank is 6' and man it was aweful. It was ... well...flexible. It got dips that filled with water; it would fill up and then syphon...fill up and then syphon...It was not very quiet. I would really think before I did this. Finally it just fell off the overflow from all the movement.

I have ABS pipe in a similar configuration on one end of my tank...it does get quieter...Also I did not use glue so it comes apart nicely when you want to clean out the sump..

I second Ross's comments ... flex line for the drainage is not necessarily optimal. I tried that too with my first sump. If it had a low spot, then what would happen is it would slow the flow ever so slightly, then the pressure would build up and release and the flow would speed up again. It sounded like a toilet being flushed, every 30 seconds to 2 minutes. It was horrible.

The best results I've ever had with reducing noise, involved having the water "slide" into the sump at a 45 degree angle. I've tried every combination of 90 degree bend, above and below waterline, etc. etc. etc. and while some of that does work to reduce noise, for me I found the best results with a 45 degree angle. Not always easy or practical though, but something to ponder perhaps...