Ion Berkley.
The handheld most likely doesn't hear the noise source because the internal design of the radio is completely different and so it's potentially sensitive to different frequencies of noise.
Ion is right but I think the only way you’ll pin down the source of the problem is to use a radio as a mobile detector. If you can get hold of a connector to the back of one of your radios then wire up only the power lines, the phone output (terminated in a phone socket) and the antenna with six feet of good quality 50 Ohm coax. Plug in a decent antenna to the coax. Tie the antenna onto a length of broomstick.
Power the radio with a battery and take it aboard. In flight, have a passenger listen in to the headset while moving the antenna around the cabin, windows, and into the foot-wells.
If this doesn’t exactly pin anything down it may give you an area for the source of interference.