Faint Knocking Noise - suspension

I have a strange knocking noise coming from I think the driver's front wheel. It is most notable when driving off and at low speeds on smooth and broken tarmarc. Sounds like a rubber ball in a plastic bottle, or liquid sloshing.

Little ramdom, dealer could not locate anything but will be taking the car back to them. Any ideas on what could be wrong with the suspension so I can point them in the right direction? I can replicate the noise when I open the hood and bouce the car up and down!!