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.
The buzz is still occurs sometimes, when the battery is really low, but mostly the problem has disappeared. The obvious solution is to charge the battery every now and then.
Even charging the battery did not help anymore and the buzz was getting too annoying. When staying outdoors there was no problem but indoors (or whenever the signal was a bit weaker) the buzz returned. It’s as if whenever the phone needs to boost its sending signal some component starts interfering.
Well, I found a page with the original instructions for insulating the power wire, but also a scanned drawing of insulating the loudspeaker at the back of the phone. I followed these instructions, and the sound was again clear as crystal.
My theory now is that it wasn’t mandatory to add this new insulation, but there’s something that’s collecting inside the Treo that’s purged everytime I open it and fiddle around. My suspicions turn to my magnetic phone holder in my car. It may be that keeping the Treo in it will induce electric charges in the battery and also the aluminium folios that I’ve been adding, and every time I reopen the Treo the charges are nullified. Well, I’ll need to test my theory…