I'm leaning towards a graveyard that used to be where you house was... they moved the headstones, BUT DIDN'T MOVE THE GRAVES!! Happens all the time.
The transformer could be in the basement or crawlspace. If it's getting louder, it probably needs to be replaced.
I don't want to alarm anyone, but you all recall what happens to the person who goes into the basement or some other small, unlit (dang lights aren't working!) place in the haunted house? Hire some homeless person or ask your in-laws to do that task for you.