Why Is My Phone Producing a Cracking And Buzzing Sound?

I went to fix my charging port, and after the job is done, I discovered that my phone is producing a cracking and buzzing sound, every time I played a song, movie, or on call and every other sound-related activity on the phone. It only works well when connected to earphones or Bluetooth speakers. The phone speaker was changed by the engineer, but it's still the same. Please what could be wrong?