boomer, i think the vinyl tubing to feed the pump is a good idea. from what i understand the more bends and kinks you have on the inlet side the more likely you are to have cavitation. It almost is like reverse head pressure kinda.. So vinly tubing would not have the 90 degree bends that PVC does. However the head pressure on the outflow side should not be an issue at all.
Jeepers, did you put a ball valve on the outflow side of the pump? if you did try slowly closing it and if the sound stops after youve tweaked the ball valve a bit, then it is 100% without a doubt cavitation and, as you would now beable to see the head pressure is not the issue, as weve just increased the head pressure dramatically by closing a ball valve 1/4 (or more) of the way. (this is exactally what my closed loop did, FYI) So the removal of the squids and the reduction in head pressure will likely make your pump LOUDER, until you fix your cavitation issue. The other possiblility is that you have an air leak in your PVC somewhere. did you glue all the pieces together? even the ones over the tank that are out of watter? Teflon tape around the pump fittings? I have heard of all of these things causing air leaks and producing a smiliar sound and effect as cavitation... But it sounds like you are dealing with cavitation if i had to guess. i would try vinly tubing on the intake like boomer mentioned, maybe a little larger than 1" if you can squeeze it in there.