My neighbor's lawnmower taught me about soundproofing

I was trying to record a podcast in my backyard studio last week when my neighbor fired up his lawnmower. The noise blasted through the walls and ruined my audio. I knew I had to fix this problem. I looked online for cheap ways to block noise. I ended up putting thick curtains on the windows and foam pads on the walls. It helped a lot, but I still hear low rumbles sometimes. Now I schedule my recordings for early mornings when it's quiet. Has anyone else dealt with outside noise in their studio? What did you try?