crackling noise from underneath

my 02 venture makes a crackling noise, similiar to rain on a tin roof, underneath the vehicle (in the middle, slightly behind front seats) when I drive over 40. does not happen below 40. pretty loud. first thought was sand on road being thrown up against undercarriage, but no sand on road. help!

You might check exhaust system for loose parts or a loose heat shield. Maybe there is a resonance that is making the sound.
If it is mechanical, just keep driving until something falls out from underneath the van. Then pick it up and take it to the auto parts store and announce, "I need a new one of these."