Hello to all.
A few days ago I also did that work because my Bitx40 also suffered from a strong microphone that was born in the AF Amp and surroundings.
I removed the ceramic capacitors C115 and C116 in the AF Amp circuit and also C120 and C122 in the MIC Amp circuit.
All of them were replaced by 1uF/35V Electrolytic capacitors and the annoying effect disappeared.
It is very important to respect the correct polarity of the electrolitic caps.
To remove the original caps, simply apply a large amount of tin on both sides at the same time to remove it, then dislodge the tin with the desoldering braid to place the new one.
I hope I have explained it correctly, English is not my native language.
Good luck and enjoy it.