How to open a power unit for Bissell Big Green Clean Machine? 4 unconventional deep well fasteners..big pain.?

My Bissell Big Green Clean Machine of 10 yrs. has low suction yet motor is fine. Tubes are clear, motor rpm fine. No air leaks anywhere. I think the impeller may be fouled with fiber or dog hair. How then to open the stupid power unit? There are 4 holes in the gray plastic module, deep welled to ensure the poor customer great difficulty opening it, forcing you to get it "serviced" for megabucks. Shining flashlight down a hole I see a metalic "something" like a rivet..not a slot or phillips screw, allen, hex bolt etc.. How do I open that power unit? I don't want to pay some cluck 35 bucks to clean dog hair out of an impeller.