There is a known squealing issue with some models of ICOM handhelds. I've encountered it on an ICOM A5, but it doesn't occur on my A14 even with the same headset.
There is a set of toroids ("chokes") that the mike cable wraps around, and it supposedly will help with the squealing. They're $5.35 from Aircraft Spruce:
https://www.aircraftspruce.com/catal.../icoma6a24.php
(If the link doesn't work, look up "ICOM A6 / A24 HEADSET NOISE MODIFICATION FERRITE CORE".)
Mind you, I've tried endless variations of these, and it doesn't help my A5. But buy a couple and try combinations of wraps.
One thing you MIGHT try is to reduce the sidetone volume on the radio. On my A5, you push the transmit button, then turn down the normal volume knob/button.
Folks should also understand one other key point: When you use a headset adapter on an ICOM handheld, you MUST have a separate push-to-talk switch. If you push the "transmit" button on the radio itself, it uses the built-in microphone.
Ron Wanttaja