Inspiron 13 7380 intermittent audio problem

I've had this machine for less than one month and there is an intermittent audio problem. Occasionally, seemingly at random, the bass frequencies will drop out, leaving only middle and upper frequencies, e.g. when playing a Youtube music video. It's the kind of effect one would get in a system with a sub woofer that failed. I can't provoke it, and it might not do it for days, so I've been unable to determine whether it also affects headphone audio, which has been fine so far. When it does occur, it lasts for no more than three or four seconds before resolving itself. The duration is consistent, and it's never produced two instances in a row. There is no other buzzing, crackling or clicking with the audio. All of the Windows and Dell software is up-to-date, and Dell diagnostics show no hardware or software problems. Running Windows 10 Home 64 bit. Any suggestions would be welcomed. Thanks