Here is a reply I got from a certified Line 6 service tech:
From everything you’ve written I’m pretty positive the sand/rain was the cause of the static. More than likely it has affected the pcba boards in there somehow, and they would need to be replaced. The problem is these are older units and the boards may or may not be available, depending on what it needs.
*** You would spend more fixing it then simply buying another one from the used marketplace.
So now I've got several thoughts going on:
* single guitar pedals on a pedal board
* tc helicon vocal processor of some sort
* find a used H500 replacement, upload my saved patches from my computer and make this simple, but start exploring those other 2 options.
powbob's post in HD500 Screaming Static Noise all of a sudden. was marked as the answer