Knocking Noise from under Car !
2004 Vauxhall Astra 1.6

could be your anti roll bar bushes worn have you checked to see if springs are ok are your shocks leaking ?

the astras are very common for wearing out suspension arm bushes you can get these from any motor factors as they are common and that will be your problem solved buy the whole arm as this is less time consuming when replacing ie labour unless you are able to do the job yourself