Further research reveals that this is some audio pop error that is fixed when TX is on, but when someone wants to practice morse, there's that anoying pop again.
Reading through the forum, I found that some suggest that audio pop from TX/RX switching comes from charging mic preamp/turning off receive circuit and vice versa.
But, during TX off mode, receive circuit is always on and mic preamp is always off, but there is that audio pop again.
This time, pop comes from CLK#0 and CW_KEY Raduino pins.
