Is it possible that the plate was flexing downward with the suction until the vacuum broke (by sucking air)and then popped back up which could case the plate to spring up and down? This could cause a vibration that resonated down the tube. Water carries vibrations really well. Consider putting a stiffener (or two) lengthwise from the u-bolts out to the end of the plate (in line with the pipe). A couple pieces of angle iron (1-1/2 to 2" by 1/4" thick) drilled to match the u-bolts and then drilled and bolted to the end of the plate would take any flex out of the plate itself.


Fish on!,
Noel