How can I mitigate humming from a streetside electrical transformer?
That noise can really get annoying. 

Hate to say it, but I'm not sure there is much you can do about it in your house. However, there are steps the utility company can take to mitigate it. 

Here is a writeup of what you are dealing with:
https://electrical-engineering-portal.com/transformer-irritating-hum

They may have to adjust how it is mounted, or worst case (for them) change the transformer as some are noisier than others. You will probably have to be persistent.