Replacing audio-jack (charging port) didn't solve the sound problem

I got a second hand iPhone SE and the headphone jack wasn’t working - there was hissing and other noises. Wobbling the headphone plug sometimes helped a little, but then it quickly went back to hissing again. I tried several headphones - same result. Then I decided to change the whole audio-jack/charging port assembly. Luckily I have another iPhone SE with a working headphone jack. I replaced the unit, but the sound problem didn’t go away - same hissing and crackling. I then put back the original part, and now even wobbling the plug doesn’t do anything - it’s now just noises and hissing. I wonder - if changing the headphones jack unit doesn’t help, what else could be wrong?