Before anything DFPlayer related comes into play here..
What sound files/fonts are you using? Home made? If so.. that is probably the first issue.
All files used needs to be edited to that are seamless, including swings..etc..
That being said.. the DFPlayer module itself it just a 'generic' playback module... Its not very fast, or clean.
And for a seamless playback of a looping sound (like the hum).. .. you need to have nothing else going on.. no serial outputs.. nothing. and then it may be 'ok'..