very likely the wishbones weakness.
the upper wishbones tend to be the creakers, the lower wishbones tend to be knockers, but not always.
to be on the sure side, unless you have history that shows som parts being done previously, i would recomend doing both side uppers, both side lowers, anti roll bar drop links and D bushes all in one go, as youre in the area for the whole thing, and doing it all at once helps save time and labour money in the long run if you're not doing it yourself.
with some hunting, you should find all those parts readily available for about £200, but the nice tight silent drive afterwards is quite rewarding.
i'm not a qualified mechanic by trade, more a self learner from wanting to know how my cars are screwed together, but i could do that little lot in a decent day now having had some practice. The lower wishbones are the only bugbear really, take some lining up and getting in, even though your original one will simply fall out once undone and free! Odd!
good luck though