I have a pair of floor standing speakers and one of them has a rattle that is very pronounced at high volume and a certain frequency.  I pulled out all the drivers and was not able to find the source of the problem.  Any advice on how to find this problem would be appreciated.  Thanks.

Loose screws and fittings can cause rattling as can wire moving inside the speaker box. Nothing should be loose.       mike

Step one - take your computer or ipod and install a frequency generator application.  Then slowly sweep the speaker from 1000 Hz down to 30Hz.  As you approach 400Hz the cabinet will begin to excite.  When you hit the resonant frequency of the loose part you'll be better able to hear where it is coming from.

If the problem is not external, then remove the driver (while the speaker is playing the target frequency) and hold it in your hands away from the cabinet as far as the wires will allow.  

If the problem is in the driver, you'll be holding it.  If the problem is the cabinet you'll hear it return when the driver is re-installed.

If the problem goes away and you can't figure out why, it was a problem with the mounting screws becoming loose or the hookup wire behind the speaker was looped in such a way as to touch the back of the speaker cone.

Steve