If your compressor doesn't have them, rubber isolation feet under the bottom of the compressor also helps to reduce noise. I use the interlocking fatigue mats found at Costco or Checker Auto under my portable unit.
Coleman Powermate offers the 20 gallon, 5HP POWERBENCH Air Compressor. It's essentially a mobile workbench on wheels with air compressor and tool storage below. The compressor is fully enclosed and claimed to be quiet. I never personally heard one though.
Here's some more specs:
MAX PRESSURE 125 PSI
CFM @ 40 PSI 7.1
CFM @ 90 PSI 5.7
VOLTAGE/AMPS 120 / 230
Perhaps it will stir some ideas for your own compressor cart.