I don't see any form of mixture problem causing "knocking" noises. Misfires maybe.. misfire based rattles yes. Does it misfire?
How bout a loose catalytic convertor. Try banging on it with a rubber hammer with the car on a lift.
I base the thought on the difference between hot, cold. The catalyst takes longer to cool as it is well insulated.
Bosch Master, ASE Master, L1
33 years MB technician