When the Mic LED flickers, this means clipping is occurring; see page 23 in the manual. Also see page 40 in the manual and read about the Clip Meter, which is a small black mark that appears on the dB meter when clipping occurs. Clipping happens when the signal being recorded is momentarily greater than the hardware circuitry can handle. Instead, the hardware hard limits the strength of the signal, which causes a harsh scratchy distortion in the audio signal during playback.
The only way to avoid clipping is to reduce the recording level for the signal, which can only be done using the Rec Level button on the right side of the H4n. You can not avoid clipping by decreasing the gain in the MTR mixer, this only adjusts the sound level going to the headphone output and does not affect the strength of the recorded signal.
Even though the recording level seems to be -6 dB, it's possible for the signal to temporarily exceed the apparent recording level. This may happen for only a few peaks of the audio signal waveform, which is enough to trigger the Mic LED flickering but not enough to be visible on the dB meter. The meters typically have a fairly slow response, to make them display a short-term running average level, and to make them easy to read without the reading jumping around millisecond by millisecond.
The H4n built-in mics, like many mics, are very sensitive to wind and other forms of air motion, even something like a draft from a heating duct. This can cause a low-frequency fluttering noise in the recording which can contribute to clipping. There also can be low-frequency noises from vibration in the environment (like from people moving around), plus other background noises, that can be picked up by the unit but are not apparent to a human listener.
You can try using the foam pop filter to reduce wind noise, plus you can try using the Lo Cut feature (see page 68 in the manual) to reduce very low-frequency sounds.
- John Santic