From what I can tell- it is all air related. The more air the system pulls in the more gurgling and plumbing sound it tends to make. I have found that getting the standpipe muffled with some polywool and ensuring that no air is in the top loop is the key once the invert is drawing the water in from the tank efficiently.
That may be the key - if the overflow invert is drawing too hard it will pull air in with the water and that will constantly cause gurgling no matter how much you suck air out of the top loop with a one way valve etc. If it is too low the flow rate will be too slow/pump must be retarded.
I have found a good medium and discovered that having the 90' on the invert with a grill on it was effective rather than a vertical pipe - although some slots in a vertical would probably work well too - cause I got whirlpools when I did that.
perhaps even a length of horizontal going onto the elbow would allow the flow to normalise. I was reading in the overflow thread about various venturi systems to quieten them but mine has settled down to the sound of a running water feature rather than noisy plumbing and I am very happy with that. Getting everything airtight and the invert flowing properly tended to be my biggest issue.
As for getting that clamp out - other than removing the system and flushing it out - maybe a long wire with something big enough to push it right through?
Will
