I strongly recommend everyone put an antenna analyzer on their antennas at the end of the feedline going to the radios. Do this during the build and anytime you change something or have a radio problem. This tells truth and is a much preferred shortcut to the typical “change stuff out at random” approach.
Side note: Using an analyzer is how you make a wingtip VOR/ILS antenna work to its full potential.
Here is a nice one that can be a shared tool for Airparks, EAA chapters and such. Find an Amateur Radio Operator that knows something other than digital comms to show you how to use it.
With the little information provided, I’d guess the antenna is not the issue but I’d start there anyway as it is dirt dimple to check with an analyzer. Instead this maybe standard RFI coming from someplace yet to be determined.
A simple tool to go search for RFI is a hand held radio with a little rubber duck antenna. Sit in the plane (with the plane outside the hangar) and listen to ADIS or such. Then change to a frequency not in use. Now with all breakers open shut your master switch(s) and listen to the change in noise on the handheld. Move the handheld around looking for a increase in noise (as in your are moving it around to see if anything changes as the handheld gets closer to stuff in the panel). Now bring the non-panel stuff up one breaker at a time and repeat. After that bring up the panel one breaker at a time and repeat. Hopefully you will find the culprit (unless it is the Garmin radio itself but even is so it might show up on your handheld).
Note - do not have any external power connected to the plane for these checks. If needed charge the battery(s) after you are done.
Let us know what you find out.