Otherwise do a visual check, on the AP reporting it at 40 dB, the interfering device must be very close to that one. Like a couple of meters probably. Could be anything like an AP, provider router, end-user 4G hotspot, some kind of smart device/tv. Or download the free netspot on your laptop (assuming you do not have access to a professional survey tool like Ekahau), go to the area and walk around to see where that signal is the loudest, you must be able to find it.
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