Question: Question about string height and noise

I have a five string Bongo. And my question is that the sound of B string is much louder than that of G string. The difference is about 8-10 dB on the interface of DAW. I have set up my Bongo as FAQ says that the pickup cover is parallel with the pickguard.

Another question is when I am recording, I can hear some noise which frequency is greater than 3kHz. Is it because of low battery, or is something wrong with the electronics?

Thanks in advance!!!