Brake Gringing Noise

I have a 2000 Grand Caravan. I recently replaced the brake pads (32K miles) and had the rotors turned. After this change we have been noticing a grinding noise from the brakes. This doesn't happen all the time, it is happens randomly, but once it happens it does it several times in a row. It always happens when the brakes are depressed. I have taken it in and had the brakes inspected and they say the brakes are fine. Any thoughts on what the problem is and what to do?