I'd comment on Fayaz's answer, but I don't have enough reputation to comment. So here's my additional advice:
Along with checking hidden white space characters in
wp-config.php file and elsewhere, I'd recommend you to temporarily disable all plugins and switch to the default WordPress Theme (e.g.
Twenty Seventeen Theme). Then check if you still have the error.
If error / warning is still shown after doing the above, then definitely it's coming from core files, so follow Fayaz's instruction on removing whitespace, by creating new
However, if no error / warning is shown after doing the above, then it's not coming from
wp-config.php, rather from your theme or a plugin. In this case, enable the theme & plugins one by one & check which one causes the error.
You say you didn't install new Theme / Plugin, but it's possible that the error was there forever & only now it's showing after enabling PHP error reporting.
Note: It's also a good idea to clear browser cache before you do this test. Sometimes browsers show output from older sessions & that may confuse you while you test.