Recently I've started noticing that when I push the PTT to transmit on my ICOM A200, while on the ground, I hear a high-pitched ringing. I did a radio test and my transmissions are still 5x5, but I can hear this high pitched ringing through my headset. Strangely, I've only noticed this when transmitting with the aircraft on the ground. Could it be some kind of reflection or feedback effect?