IPad mini 4 speakers crackling & slightly distorted

Hello!

About 3 weeks ago, i noticed that the speaker on my iPad mini 4 started to produce some crackling noises when playing sounds on high volumes, especially when i play the Opening ringtone on full volume. The crackling & distortion is more noticeable when the speakers are closer to my ears. It is also more noticeable when i play piano music & songs with deep bass tones

What i have tried is restarting settings, only to no avail.

Some things to keep in mind:

-the iPad is a 16GB WiFi model and runs iOS 10.3.1

-The iPad was bought as 2ND HAND (used) on Feb 2017, but the seller said that the speakers were already replaced.

-All equalizers are turned off

-Mono audio is turned off