Hello Chuck & DJ:
Welcome to IRV2
I always enjoy the two of you and it was good to see you in AZ. The same thing happened to Donna & me.
It turned out to be the Vacuum Pump; I replaced it without to much trouble. The pumps are not cheap; I later discovered they were a GM part although I do not have the part number. You can try to get one from GM that is used on the Duramax trucks.
The unit on the 01 was located on the outside of firewall and can be reached when the Gen slide is extended.
Someone had to unscrew from inside while I held the nuts on the outside.