@Wayne-TP 

I have tested on different lamps and outlets, the bulbs always buzz regardless. It sounds like inductor coil whine in the switched mode power supply.

I tried taking a video but my phone's built-in microphone is not very good. Instead, I used an audio spectrum analizer app which shows the frequaencies at which the buzzing happens. I include links to screen photos (screen capture not possible because the buttons make noise) but they are not very good becasue my phone camera had trouble focusing on the reflective surface of the display:

Smart bulb 1, highest brightness setting: Buzzing at 996 Hz (photo)

Smart bulb 1, lowest brightness setting: Loud buzzing at 19312 Hz (maybe my cat can hear this, I can't 😀) (photo)

Smart bulb 2, highest brightness setting: Buzzing at 902 Hz (photo)

Smart bulb 2, lowest brightness setting: No buzzing at any frequency (good)

Becasue the buzzing happens at slightly different frequencies, it looks like it is a hardware problem and that different components resonate at different frequencies due to tolerances. That said, driving the LEDs differently (for example at a higher frequency outside the human hearing frequency range) could solve the problem, but if this is somthing that could be addressed in firmware.

  @RandyAndy Quick update, I just opened the other pack of two bulbs. The bulbs from this pack are definitely much quieter. Both make some buzzing noise but one of them is so faint I have to put it right next to my ear to hear it and the spectrum analyzer app cannot detect it. The other is a little bit louder but still much better than the previous two, in this case the spectrum analyzer app can detect the buzzing and it happens at a different frequency than the previous two bulbs. From these findings it is clear to me that this is a component tolerance problem. Assuming the noise is caused by an inductor, the inductor may not be properly potted, or the potting is not consistent, that's why different inductors can buzz at different frequencies.

I'm thinking I might just buy another couple of packs, select the quietest bulbs and return the rest.

I'd like to point out that my father has two Wiz smart RGB bulbs we tested yesterday and they are completely buzz-free.

  @Wayne-TP I could go the warranty route but it could be time-consuming so, insetad, I will return-buy-return-buy-... until I get 4 smart bulbs that are acceptably quiet. I've already done my part by taking the time to perform tests and report the problem to TP-Link here on the forum, and by saying that the bulbs I have tried from Wiz don't buzz. Hopefully TP-Link will fix this in future revisions of the product.

I have tested using a hardware decibel meter, you can see a video of my testa video of my test. I cannot guarantee the meter is super accurate but I can measure the dB difference between far and near the bulb: 66 dB and 37 dB, respectively, therefore the noise emissions are approximately 66 - 37 = 29 dB (greater than 23 dB).

  @Wayne-TP same here. Sound studio. Anything above 50% brightness and the buzz/whine is audible at 1.5 - 4m distance. 

  @Mully_DE I think most people won't notice because they are using the bulbs in a normal room, but in a well treated studio you can hear a wristwatch ticking.

I bought 4 bulbs (two packs of 2) and the two bulbs from one pack had the buzzing, but the other two were considerably quieter, see my earlier post. I kept the quiet ones and returned the noisy ones. If you bought them on Amazon and you don't want to waste time, you could just buy 2 or 3 boxes, test and keep the quiet ones, then return the bad ones.

Just a tip, I never run mine at 100% cold white because that turns all three LEDs (R+G+B) on at full brightness and the bulb gets very hot, which will increase the chances of failure. If you run them at 80% they will be cooler and probably give you a much longer service. If you need full-brightness it's probably better to use regular "dumb" LED bulbs for that.

  @Wayne-TP What is the current / latest hardware version for this product?