I have a 2002 Sienna with a knocking noise in the rear brakes?

When braking at low speeds, like pulling up to a stop light, there is a knocking noise coming from the rear. The noise stops when letting off the brakes. I've had the vehicle to the dealer 4 times, finally replacing the rear shoes and resurfacing the drums. On the way home from the dealer, the knocking noise returned. The noise is cylical and corresponds with the wheels turning, but is only present when braking. Any ideas on what this could be? The dealer ures me the brakes are in perfect working order and I don't notice any problem braking, just this noise that recently showed up after having the van for nearly 5 years.