Here are my results.
The main noise I get is from impeller cavitation. This sound is amplified because of the hollow V shaped volume the the input tube goes through (just above the pump/impeller)
First thing I tried was to pack this with foam, that seemed to reduce the sound well.
Thinking that sand would do a better job muffling the vibration, i pulled out the foam and added dry sand to the V. It _might_ be better than the foam, but not much better.
In the end I settled on dry sand at the bottom, and foam on top
Now the major source of sound is from the air input tube. it's resonating in the collection cup. I think I can quiet this down by adding an inline filter to the line before the collection cup.
Anyhow it is quieter now, FWIW