Why does my iPhone constantly vibrate when plugged in? No display.

So this was a phone given to me by a friend. The power button didn’t appear to work so I suggested a replacement. It did this wierd thing where it would only hold a charge to 2% when plugged in and switch itself off. So I replaced the volume flex and now there is no disaplay but when it is plugged into power, it just vibrates. It started as a contiuous vibration, but now it’s more like a phone call - intermittent. I had thought about changing the battery to address the charging issue, but I can’t seem to get my head around the vibrating situation. Any info would be helpful, thanks!