I just figured this out by reading THIS, but I inadvertently fixed the issue when I unplugged the ABS unit because it was pumping air into the brake lines and leaking.
Pump continued to run after shut down draining battery.
After husband slammed on brakes multiple times in a few minute span the abs light came on. Then the humming sound stayed on after turning off car. I figure it is ABS pump running continuously or something wrong with the relay. Will pull ABS fuse to see if that will stop the pump from running so it won't drain the battery tonight.
ABS warning light illuminated on the dash. I googled "abs light on 1991 subaru legacy" and a link (this page) said that a common problem was the relay burning up and causes the ABS pump to run continuously (even with ignition off). Haynes manual revealed the location of the ABS relay. Replaced relay with new one. Problem fixed:)
At approximately 190000 miles my 1994 Subaru Legacy 2.2L Sedan began "eating" batteries and then alternators. This resulted in about four warranty replacements of each over a period of four years.