According to previous post 1834, we think the noisemon problem is very likely caused by C2 charging. Hence we did the following modifications on a noisemon board:
1. Split R2 into two 400 Ohm resistors, with grounding between them.
2. Split C2 into two 3.3uF capacitors, with grounding between them.
We hope these modifications will provide a path for the bias current to go to ground, instead of charging up C2.