Low-Frequency Buzzing Issue with Quest 3s Built-in Speakers

It's probably more of a design issue; the built-in speakers can't handle much bass. Maybe turn the volume down a notch or use headphones with better bass response. If the problem is constant and a serious problem, you could try a warranty return, but be advised Meta will send you a refurbished unit that might be older than your 2-month old headset. You could try to get a refund, perhaps, especially if you didn't buy directly from Meta, then buy another new headset, but that depends on where you purchased from and their refund policy.

Given that it took 2 months before you even noticed the issue, I guess it is not serious, so probably just ignore it.

There is no buzzing in my Quest 3s even at highest volume. It could be firmware but there is a possibility for hardware too. Sometimes a speaker unit can come loose inside and certain frequencies can cause it to vibrate. Is there a chance the speaker arm took a hit or was the headset dropped? As a music freak playing around with audio equipment in my younger days I found that even the slightest movement caused buzzing from a loose speaker. I found this issue even on the tiny speaker on an old Samsung phone I had. I jerry rigged a bit of putty around it and the buzz was gone. I'm not suggesting it is definitely the problem and I don't advise popping it open! 😁 But, you can definitely play Quest 3s at high volume without buzzing. So something isn't right. I'd contact support if I were you as it might get worse over time if it's a hardware problem. 

Note. I meant to say if, when you are using headphones it's perfect, it's more likely to be a physical problem with the speaker, particularly if it's more noticeable just on one side. If it's both sides, it's likely firmware. You could try a reset maybe?