This is a common problem, the solution is actually called the "Treo Buzz Fix".
Earlier models of the Treo 600 had an unshielded cable that connects the battery to the main board. This cable causes loud buzzing and interference on the Treo.
If your treo is under warranty (unlikely probably...) then send it back to Palm for replacement.
If not, try the fix. The fix involves disassembling your Treo and wrapping aluminum foil around the offending cable. Here is a guide:
And here is a forum on MyTreo with high resolution pictures of the process, to help you:
This is a somewhat dangerous procedure. It is possible to damage your treo if you aren't careful. Follow all instructions EXTREMELY carefully.
Good luck! And don't break your Treo!