To be honest on that note i cant be sure then, i know on my astra i was getting a noise more like a squeak than a knock when going over bumps and was put down to anti-roll bar bushes and was a common fault, never did changed mine as yet as dealer said wont do the car any harm.
Hope youfind the problem best of luck mate.
z1asid said:
vectra 2.2 sri.deffo a knock.