clunking noise on uneven road

[webonline]

Hi All,

I've changed my rear pads a few weeks back on my Honda Accord. After few days,I noticed a clunking noise on uneven road surfaces. Its more noticeable at low speeds. If I go over a drain or a bump in the road there is a clunk coming from the back. High speed is fine. Everything appears to be tight.
When I pump the peddle stationary there is no sound. Its only when moving. Any ideas?
Thanks in advance.

[itsnotarace]

Is it one of the shims missing or rattling?

[webonline]

its more like rattling at low speeds over the bumps...