We had this problem when I was at Hoover... A LOT of back and forth emails and phone calls to the engineering department.
You have to take off the back plastic piece that holds the attachments (part of this also goes around the hose opening). I think there are 4 screws... That panel snaps off, and you can see the microphone for the dirt finder.
We had a little foam wedge that we would put in the part of the tool holder piece that went around the hose opening and butted up against the microphone. The foam wedge held the microphone firmly in place. And held it still from vibrations, which could resonate through the machine. At least, that's the explanation that we got for the EDF lights staying red. We were also told to shoot a squirt of WD-40 on the back of the microphone, where the wires connected to the microphone. No idea why... But it seemed to work most of the time.
Hope that helps.. I can take some pictures of what I'm talking about over the weekend if you can't follow my words.
~Fred