ESP8266 Community Forum

I have a chinese Wemos D1 mini board ($2 at aliexpress) and when testing an electromagnetic relay I noticed (heard actually) that when relay enabled (high state at relay's in) it generates regular continuous sounds like if was the relay's contacts. I connected it to the external 5V power supply and the sound is gone (relay is good).
The current drawn by the relay is next to none (40mA) which for the board should be no issue. Might it be USB power source interference? The sound is there regardless whether powered from USB port of USB charger.

what makes you think that 40mA is ok for a single gpio???
Or do you have any "circuitry" coming with your relay?
Try boardsearch with "relay" and you will find several threads dealing with how to connect relay to esp.

If you are trying to switch the relay power directly from the port pin (no external drive transistor) the rating for the ESP8266 is 12mA maximum. And with that it has significant voltage loss.

You are aware that they sell relay shields for the Wemos (that include a drive transistor and flyback diode)?