After years of use, some problems may present themselves. I've had loose joints in basshorns make this symptom, allowing the panels to resonate. You'd think loose joints would buzz and they usually do, but I've had 'em come apart in a way that made panels lose stiffness just enough to sound resonant.

Also, you might double check the woofer and make certain it isn't a cheap mudmotor woofer. That's half the battle right there.