[Elecraft] "Popping" noise in audio

[mikekopacki at verizon.net]

Just used my newly completed K2 for my first contest - FYBO.

There was only one thing that was not 100%.  I found that, on 40m and below, with or without a headset, there was a "popping" kind of sound in the audio when transmitting WITH THE PREAMP ON.

It was not detectable with the Preamp OFF, or on 20m, 15m or 10m with the Preamp ON.

Filter settings were 500 Hz for FL1 and AF1 in all tests.

I did not hear it in SSB mode.

Any ideas?  Thanks from
Mike, NJ2OM