welcome to Loonyzone.
If yu are getting a knocking from the rear, it is likely to be rear diff mountings - an easy job - speshully the front one. Or chassis cracked/rear subframe mounting broken - also fairly easily fixed....
however - first option has to be take it back from where you got it and demand yo money back as car was sold "unfit for purpose". Threaten trading standards if it isnt fixed at no cost to yoself.
then go buy a TD4 or a decent motor.
