@ErkanB 

Could be something inside the adapter or possibly another appliance. If the adapter just started doing it I would look to see what else was plugged in around your residence. If nothing was then probably a loose component internally.

@Tony 

Nothing new has been plugged in recently in my residence.

I guess I shouldn't open it up and search for the loose component as I think it is still under warranty. 

Thank you.

@ErkanB 

Ya, definitely do not open it up. Follow up with support and they will let you know what options are availble.

@ErkanB I have in total 4 units of AV600 installed in my house and 2 of them started to have the same exact problem a few weeks ago. One of them started to keep that annoying squealing noise and stopped working completely. I could not pair that unit anymore or create any kind of connection to it. I contacted the seller and had it replaced under warranty. The other unit makes that same noise. It still works somehow, but the connection is not stable. When I put a bit more traffic to that unit using WiFi (like running Speedtest), the squealing noise becomes louder for a moment until the connection is dropped and the unit is reset (all the leds go off for a moment and start to blink until the unit is accessible again). The problem occurs only with WiFi connection. If I run Speedtest with a cable connection to that same unit, the noise is still there, but the connection is stable and the unit is not reset. Anyhown I'll ask to have that unit replaced also.

Maybe there is weak/faulty component used in a particular manufacturing lot and now it starts to break at the same time. 

Same problem here. Bought mine in 2018, so there is still time left under the warranty. Did you get a replacement?

@MrCooky Yes, I got them both replaced by Amazon. Also the second unit broke completely within a couple of days from my post. 

My units were bought on December 2018.  

@ErkanB Mine, purchased in November 2018 (so not under warranty )  just started doing the same thing.  Bad batch??!!

@ErkanB Mine has just started doing the same thing. I bought it from Takealot in South Africa in August 2018, so it did work perfectly for just over 3 years. I have no idea what warranty tp-link offer, I assume 3 years is the best case scenario. Takealot only offer 6 months. I could winge about planned obsolescence, but at this point I'm just gonna open the thing up and see what's wrong with it. Can anyone give me any pointers?

To be clear, it's making a high pitched whining, and neither the LAN port nor the wifi extender are currently working. They both were working before. I did reset the unit, re-pair the powerline, and resync to the wifi router. No joy.

@Bles I got it replaced under the warranty period. Most probably a bad batch from 2018.

@ErkanB I solved the problem changing the capacitor 470uf 16v