Exact same situation here: after a rebuild, it kept vibrating as long as tension was there (at startup, and when screen is on - and a little while after). I followed Eduards Blumbergs advices : I've reset this metal pin to its initial position (I suppose it's meant to be parallel), no result.
Then, with the phone powered on and opened, I tried to unscrew a bit the bridge-looking metal piece which links the vibrator to the other contact, and the vibrator stopped totally. I tightened it back, and it went crazy again.
Then, I unscrewed a bit only the top left screw, the one on the vibrator, until it stops, put my phone on silent mode, and with another phone, I call my iPhone SE, and it vibrated only when needed!
I guess too much pressure on it was forcing the contact on the power bracket under it.
Thanks for all your input, glad I found this thread, which is a lot more accurate and technical than all other google results (“my phone fell in water and keeps vibrating, help”)