Rattle under dashboard when I hit a bumps

2013 BRZ. This is a new noise, started a few months back. Something is loose and rattling up under the dashboard just above and to the left of the steering wheel. Sounds substantial. Rough road, or bumps set it off.
No idea what it is and don't feel like pulling the entire dash off to find it. Anybody have any ideas?
Car only has about 30K miles on it (replaced the tires back in December - not because of wear, but they were cracking from age - obviously not my daily driver).