OK, the buzz returned, but it only manifests itself when the battery is low on charge. A couple of days ago I reopened the Treo to check that the original aluminium patch is still there, and it is. OK, I took out a couple of slivers of aluminium foil and surrounded the entire power wire with them, so that it sits tightly around the thing and – most importantly – between the power cord and the electricity.

Results so far: The buzz is gone even when the battery is already complaining and asking for a recharge. However, by tweaking the antenna I can generate the buzz, or make it go away. Apparently the antenna contact has some problems. Will need to investigate further.

This article details how to fix the static buzz problem of the Treo 600. The problem stems from an unprotected power wire inside the Treo, and it can be fixed quite easily without paying for maintenance.

My Treo started to have symptoms sometime last summer – the people on the other end of the mobile call complained of static noise that nearly prevented them from hearing me. This was a major nuisance, of course. Sometimes it helped if I moved closer to a window to get a better reception. Gradually I noticed that the amount of power left in the battery affected the problem – when the battery was low, the problem appeared more often.

From the Palmone FAQ pages I read that it helps to have a full charge and a good reception. I did not know what was the cause, since I’d done lots of stuff (firmware update, dropping the phone on the ground, damaging it, having the antenna a bit loose, installing lots of behaviour-altering software…). When I finally did a Google on the subject, I found clear
instructions in English and in French on how to rectify the problem.

Apparently the power cord that connects the battery to the system has four wires and they are the cause of the static interference. One of the guides said that twisting the wire ends a couple of times (to achieve in essence the same protection as in twisted-pair cablind (you know, phone lines and the standard network cabling)) helps, and a Scottsman advised that wrapping the wire inside tin foil helps also. I did both of them.

As of now (just two days after the operation), I haven’t had any noise problems. But I’ll have to wait and see how things play out.

UPDATE: Reports from people I’ve talked to indicate a significantly clearer sound and no buzz. Excellent!

An excellent French article on opening the Treo and twisting the cable has very good pictures so even if you don’t know any French you can see how things happen. An English guide to wrapping the tin foil has good pictures as well, but doesn’t show the details of opening the case in as much detail. So read them both and have them open while you operate on your Treo.

Here’s a quick summary:

1. Do a full backup of the Treo. Remove the SD card, the SIM card and the stylus.
2. Remove the screw protectors with a wooden toothpick so as not to damage them.
3. Unscrew the screws with a number 6 torque (star-shaped end).
4. Remove the antenna.
5. Slide a credit card into the crevice in the side of the case and slide up and down, separating the back and front sides. Repeat on both sides and twist the card a bit to pry the sides loose.
6. Open by holding the screen downwards and lifting the back, starting from the top, separating the bottom part last.
7. Pull out the battery wire. Make a note of which way the connectors go into their sockets.
8. Twist the wire maybe four rotations.
9. Wrap a small piece of tin foil (15mm * 30mm) around the twisted wire and add a small strip of adhesive to keep it in place.
10. Reconnect the wire. The foil will easily break, so be careful. Also remember that as soon as you connect the wire, your Treo will have power, will reset and start with the tutorial and preliminary setup. At this point you can simply press the power button to shut down the screen.
11. If you drop the longish black rubber pad from inside your Treo, it doesn’t break anything, but having it does give you that luxury feeling when sliding the stylus in and out of its holster. The two documents I linked to did not contain instructions on this. But the correct place to put the rubber pad is just below the camera eye, under the rim of the green circuit
    board. When you place the rubber pad under the circuit board, the small nibble at one end goes towards the top of the phone, under the board. Push gently but firmly to set the pad properly in its place.

    UPDATE: Here’a two images showing the rubber pad and its correct placement. Thanks to Claude
    Morin for the images!

12. You might want to blow out excess dust and dirt thay may have been gathering inside your Treo.
13. Replace the back cover, first making sure the bottom is properly positioned and then swiveling the rest in place. Everything should click satisfactorily.
14. Screws in, place the caps in place, connect the antenna, insert the SIM and SD cards.
15. Push the power button and complete the primary setup. Set the language and the date properly.
16. Connect to your computer and restore the contents from your backup. Done!

Credits go to XiaoBin and Ablivio. Thanks for the instructions and the pictures!