This is a user community on which Sonos has limited presence. If you suspect a technical error, it is better to call Sonos directly.

Do you have small kids or pets that may dropped something inside of it?

Yea mate have a little 1 I've been trying to figure out if anything is in there can't here nothing 

It is when the sub goes deep into base it goes distorted/rattling sound 

If you can pick the sub up and turn upside down to see if anything fall out. It may require two people as the sub is heavy and awkward.

Mate you were bloody right ✅️ there was a decent sized figurine stuck in there eh it was lodged pretty good got some whipper snipper line trimmer tied in half and then went to work 1 side was clear and easy to get in the blocked side was giving me trouble straight away but the line trimmer did the job 

I shall see tomorrow  

You can make a cover for the Sub out of speaker cloth, anything from a simple tube that slips over it to a fancy fitted cover. Not 100% kid proof but good at keeping out bugs and dust here.

You can buy one too but the prices they want compared to a couple yards of speaker grill cloth was not a selling point.