Water Pump Insulation
The pump in my Outback isn't terribly loud, but none the less I figured an easy modification of adding some 2" foam would probably make things even better.
I cut the 2" foam to fit below the water pump then added washers above and below the rubber feet so when I screwed it in it wouldn't sink into the foam.
I then cut foam pieces to surround the sides. The black box in the upper right corner houses a 110v outlet, though it was attached at the top it had the potential to vibrate so a bit of duct tape kept it firmly in place. 
Since I didn't want to screw the seat into place I ran the edge with duct tape so it would sit and eliminate any potential vibration.
Tests at home seemed to quiet the pump even more. Real test is the next trip when the kids get up in the middle of the night to use the bathroom. I'll update the page after this weekends trip.