I power my ICOM A5 off the airplane power supply and did not get a squeal for almost ten years. Even now, it only squeals with a certain headset configuration. If I use an 8 ohm headset or a 300-ohm without an impedance matcher, it doesn't squeal.
For power, I used a "battery eliminator," a pack that attaches to the back of the radio like the standard battery pack and has a cable to attach to aircraft power
Ron Wanttaja