It will be the anti roll bar bushes, it's a common problem. My car had the exact same problem, drove me mad for 6 months. Eventually found the cause by me lying under the car and someone violently rocking the car, you can feel the anti roll bar erm.. what's the word.. Moving and making its bunpity noise.
Really easy to swap. There is one each side, just take the wheel off and with a 13mm socket undo the 2 bolts holding them in and swap. Mine were about Ł7 from the dealer.
Don't go by what the old bushes look like, mine looked in perfect condition but were ****ed