Some Treo 600 devices produce a loud buzzing sound which makes the phone almost unusable. It can be heard by the person who calls a Treo but not by the Treo owner himself. The buzz appears in normal phone mode (not speakerphone) and often under certain reception conditions only. Some conjecture that the buzzing caused by crosstalk from the GSM unit to other parts of the device.
If your device is still under warranty, it should be replaced through normal warrenty procedures. The instructions below will void your warrenty, and TreoCentral, nor RolloM, assume any responsibility for unintended consequences.
The procedure to remove the buzz was published on Treocentral first by IIRC, Nobackup and/or Milan. You can find an illustrated version at http://treshaut.free.fr/images/treoinside.jpg.
To fix the buzz, you carefully open the treo using a Torx 6 screwdriver, disconnect the black battery cable from the main board, twist it as many times as possible (carefully, no force!), connect it again, and wrap some cigarette paper (folded together in order that the metal part is inside for reasons of insulation) around the cable. Then you put another part of cigarette paper (also folded together) over the speaker (inside one of the case shells) and finally close the Treo again.
Careful: Disconnecting the battery cable completely erases the memory. Synchronise your Treo before you apply these steps!