This is normal.....
Parasitic oscillation is an undesirable oscillation caused by feedback.The problem occurs notably in audio and other electronic amplifiers.
Parasitic oscillation is undesirable for several reasons. The oscillations may radiate from the circuit, causing interference to other devices. The oscillations waste power and may cause undesirable heating. For example, an audio power amplifier that goes into parasitic oscillation may generate enough power to damage connected speakers. A circuit that is oscillating will not amplify linearly, so desired signals passing through the stage will be distorted. In digital circuits, parasitic oscillations may only occur on particular logic transitions and may result in erratic operation of subsequent stages; for example, a counter stage may see many spurious pulses and count erratically.
If you insist to speak from inside PAX area you need one extra
piece of electronic kind of phase inverter in line after your mike, or you need to manually reduce volume on PA on appropriate level before parasite oscillation starts. Tip: Go and talk from cockpit....