Galaxy s8+ crackling speakers??

Hey there! I'm sorry your S8+ is giving you a hard time.  I did a little research, and it appears that there are other folks who've also been having similar issues with the audio on their Galaxy.  Have you tried putting your phone into safe mode to see if the crackling sound persists?  You can do this by:

1. Turn the device off.
2. Press and hold the Power key past the model name screen.
3. When "SAMSUNG" appears on the screen, release the Power key.
4. Immediately after releasing the Power key, press and hold the Volume down key.
5. Continue to hold the Volume down key until the device finishes restarting.
6. Safe mode will display in the bottom left corner of the screen.
7. Release the Volume down key when you see Safe Mode.

If you don't hear the sound in safe mode, it's likely an app causing the issue.  You will want to think back to when the sound first started and if there were any new apps downloaded about that time. 

You might also want to try wiping the cache partition:

1. Turn off the device.
2. Press and hold the Volume Up key and the Bixby key, then press and hold the Power key.
3. When the Android logo displays, release all keys ('Installing system update' will show for about 30 - 60 seconds before showing the Android system recovery menu options).
4. Press the Volume down key several times to highlight “wipe cache partition.”
5. Press the Power key to select.
6. Press the Volume down key to highlight “yes” and press the Power key to select.
7. When the wipe cache partition is complete, “Reboot system now” is highlighted.
8. Press the Power key to restart the device.

The last step, if all else fails, is a full factory reset (after you make sure your information, contacts, photos, etc is backed up!).

1. Settings menu and tap General Management > Reset > Factory data reset.
2. Tap Reset.
3. If you have screen lock turned on, enter your credentials.
4. Tap Continue.
5. Tap Delete all.

Finally, if you still hear the sound, you'll want to reach out to T-Mobile via 611, the "message us" link on this page, or T-Force via Facebook or Twitter because you'll need to process a warranty exchange (provided that it's a T-Mobile device and still covered under warranty).