After LCD Replacement iPhone just vibrates when plugged in?

Hi,

So, I've replaced a decent amount of iPhone screens in my lifetime and never had any issues. A friend of mine asked me to replace their iPhone screen for them, so they brought their iPhone over, and I tested to see if any of their dents would keep me from doing a good replacement.

Anyway, I had a full front assembly that wasn't in use as I was waiting for a new battery to come in for that device.

So, I plugged it into their phone, and after doing the replacement I plugged it into the charger, and the phone would constantly vibrate every few seconds, or if the sound was on it would constantly make the "bong noise". So we tried his cracked front assembly again, and had the same issue. We tried this over and over a few times, even tried different devices, and had the same issue. Until finally we got it to work properly.

I just feel as though I must be doing something wrong? I have a couple devices here I'm trying to reconnect the front assembly and having the same issue. I'd prefer to not sell them for parts. But I keep trying over and over again with no luck :-/ - Help meee