suspension

hi guys, iv just put my 6n polo on coilovers, the tracking has not yet been sorted, and I'm having some problems...When i accelerate its smooth but as a let off the trottle the car vibrates, alot, a sort off banging/judering. It didnt do it at first but after 10/15 minutes of driving it got louder and louder. It sounds like its coming from the wheels or under the hood, towards the front off the car anyway. Any idear what this could be? or is it as simple as the tracking is not right? Any idears would be amsome, thanks.