Time to put it up on jackstands and crawl around underneath pulling and wiggiling anything you can see. Usually knocking from bumps is some loose hardware like...
Mufflers, exhaust hangers, splash/rock shields, loose suspension parts, engine and tranny mounts etc. Once I found (on a different car) that the exhaust had some resonance weights(I guess) that hung from the pipes and one was loose and knocking the floor boards.
Try to localize where the sound is coming from. Is it a thump on the floor? A sound from one of the corners? Under the hood?
Drive with your windows closed and with them open to see if you can hear and locate the sound better. Driving near walls or under quiet bridges helps reflect the sound back so you can tell which side of the car it is on.
Good Luck,
Jim Henderson