Yeah, as Welshpug says, i'd be surprised if it is the arb bushes after only five years. I'm still using an original set at over 135k miles, nearly 50k of which has been pure abuse from me.
Is it just a single knock when it happens? Sounds more like balljoints to me, but could be a lot of things unfortunately. Best bet is to get under there with a torch and lever bar and see what you can see. If it is arb bushes you will be able to move the arb back and forth in them with relative ease.
How many miles do you do? Wishbones really shouldn't need replacing that often, at least not unless you're buying the nastiest pattern parts.
Is it just a single knock when it happens? Sounds more like balljoints to me, but could be a lot of things unfortunately. Best bet is to get under there with a torch and lever bar and see what you can see. If it is arb bushes you will be able to move the arb back and forth in them with relative ease.
How many miles do you do? Wishbones really shouldn't need replacing that often, at least not unless you're buying the nastiest pattern parts.