problem with audio comment--background hiss

I am putting audio comments on a scanned pdf. When I play back the comment there is significant background hiss. I have changed the microphone volume, that does not remove the hiss. I have tried different input microphones (internal computer, and external Jabra), that does not remove the hiss. I have recorded the audio comment on two different laptops, and they have the same problem with background hiss.

I checked and recorded a note on a ppt file, and there was no background hiss. So it is something inherent to Acrobat Pro DC. 

Anyone have any suggestions? 

Thanks,

Bob