good ideas
Pete has good ideas. Folks would be surprised at the parts cost, even in high dollar headsets. The microphones are mostly cheap (as in about two bucks) condenser elements. They do have a proper side as Pete mentioned. And they are ported to allow for simple noise reduction. If you swap out headsets and check your side tone for a change, you can rule out a Garmin problem very quickly. If it isn't the mic orientation, check for a very small level screw adjustment. Lightspeed has one under a small plastic cover on the mic. Turning it down a bit might just do the trick.
If it turns out to be the radio, it might need a factory calibration. Mic level, and deviation and amplitude are factory spec adjustments and might have drifted. Many are set in software now, rather than adjustable pots. A good bench tech can quickly look at the output on a scope or spectrum analyzer. Good radio coms are almost as important as full gas tanks. IMHO