Hello Christian,
i would go with Gelbster's advice.
Another possibility could be a drivetrain / exhaust part that contacts the chassis when you go off the gas pedal. That means the front of the drive train moves down and the rear including exhaust system moves up.
A working DMF doesn't generate vibration. It reduces vibration. Please take a look:
https://en.wikipedia.org/wiki/Dual_mass_flywheel
Regards, Markus