Hi all i have a small problem....

When driving over say not so even roads with man hole covers and the poorly patched up roads we have, i get a knocking noise from underneath the car near the transmission tunnel i have jacked it up and couldnt find anything loose.
?? Could it be something to do with the prop ?? do they have rubber mounts ??
Gearbox mountings ???
I really am lost as i can fix remidy most things..

Thanks in advance.