Just a thought, without trying to sound too obvious, but have you checked the spare and the jack area?
I had the same thing happen years ago. Drove me crazy. Big bumps this metallic bang bang bang, smaller ones nothing. It turned out it was the car jack. It got away from its holder.
The other thought I had was a broken or missing exhaust hanger. They are the rubber doughnuts that suspend the exhaust system. Even with one missing or broken, most of the time there is no noise.
It is only when there is a good motion will it travel upwards and whack the underside of the car. Small oscillations won't do this.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)