Yes, the 318i is an e-46.
I will remove the back seat and check but I would be surprised to find anything. This sound just started out of nowhere. The sound is like there is something loose somewhere in the back. It is like a clunk. However it only occurs when I accelerate or when the car is going round a bend and then it is quite loud. The mechanic said that the sound comes from the barrel spring around the rear shocks. He added some rubber spaces at the upper end of the rear shock mounts and assure me the sound would be gone. It did disappear for a couple of days but then came back.
I thought of replacing the shocks but the mechanic thought it would not solve the problem. Could the barrel spring be the problem? Has it become weaker? What is a cracked subframe? If it is the subframe that is cracked, how do I verify this? Our roads are pretty rough.