harmony navigator feedback

You may have an infinite MIDI feedback through a loopback driver (IAC on Mac, LoopBe or similar on Windows). Notes being played to your synth or DAW return back to HN2 and get detected as chords, then play again, come back, ... you know.

Make sure you disable the loopback port's input switch in Audio & MIDI Setup window.

1. Open Audio & MIDI Setup window and open the "Ports" tab.
2. Open the "MIDI" tab.
3. You should see something like "IAC Driver" or "1. Internal MIDI" listed. These are loopback drivers. Disable "MIDI Input" on them (find the switch to the right, next to the faders).
4. "MIDI Input" (the red box) should only be enabled for external keyboards.

If you are using a hardware synth, the synth may also "fire back" all midi data to HN2. Disable this with the synth. Also turn "Local Off" (a midi setting that causes the synth to play only MIDI input, not the local keyboard).

this excessive feedback and distortion when I click on a chord

How does it sound? Is it just a distorted audio signal, or a stutter effect?

Are you on Mac or Windows? Which OS version?

If it's distorted audio, please try using different sounds for the global instruments, or lowering their volume. You find this in the Audio & MIDI Setup window on the "Global Instruments" tab.

HN2 works fine on 10.6.8 here. The QuickTime synth is also quite ok on Snow Leopard, It got worst and almost unusable on 10.7 and 10.8.

You may have a general sound setup issue with your Mac.

What happens when you play standard MIDI files with the QuickTime player?

Try pressing the Reset button in Audio & MIDI Setup window, next to the transport buttons at the bottom. That re-inits the QuickTime synth.