I’ve read that more people have a problem with a buzz from the existing doorbell, that’s why my experiences with this.
I had the problem that my video doorbell (T8210, battery and connected to transformer) made my existing electric doorbell buzz after switching from an 8v (8va) transformer to 12v (18va). I measure 4 volts on the existing bell.
After checking the wiring and resetting the eufy doorbell, the problem remained the same. I then decided to leave it as is and to my surprise the bell was silent after about 3 hours! Problem solved.
Was it coincidental that the 4v is exactly the difference between the old and new transformer?
Let’s hope it stays that way.