Finally caught up...
Ok, John Noble whistles in some episode(s) of Fringe, so that makes me think the whistling person whose face we don't see is Fields.
Burt having been with Lumon for 20 years and being like Attila the Hun makes me think he had to do with Lumon's early testing of severance.
Why TF would anyone invite the person who was screaming at the door over for dinner? I think Burt invited Irving to get him out of his apartment so Drummond could sneak in and snoop.
Is it possible that Irving is actually Burt's old Lumon partner?