As audio electronics power-up and power-down clicks and thumps are common. Various techniques are available to the designers to suppress or eliminate the clicks, pops, and thumps. Some techniques, while very effective, will impact the long term reliability and ultimate audio quality. The SONOS approach yields the best reliability and audio quality, but there will almost always be some sort of “click” or “thump”. Mostly the clicks and thumps will be inaudible, but the magnitude of the clicks is subject to production variation. Quiet listening rooms and very sensitive listeners increase the risk of the clicks being audible. Many users will never hear any clicks because they immediately leave the room after shutting down.

My user name is a giggle because I’m over the top sensitive to miscellaneous noises. Fortunately, I’ve never had an issue with SUB clicks. From my seat I cannot know if your click is due to simple production tolerances or an existing or pending fault. If I put my ear next to the SUB port, I can sometimes notice when SUB shuts down — mostly due to residual amplifier noise stopping.

Unfortunately, once you have become sensitive to the clicks, they will never go away — you’ll wait for them. Since this is a new unit, take this up with SONOS support. This will likely be a tricky conversation, depending on the agent. For the short term, you are probably within the return window. It’s the luck of the draw. A replacement unit may exhibit a quieter or inaudible click.

I can tell you that my sub 4 does the same thing … it’s quite low though :-)

Thank you both! I was hoping for some reassurance from Sonos support but they flat out said that any noise like that is not normal so I’ve sent it off for a return. A shame because it didn’t really bother me that much until they said that it was not normal 🥲. Also it’s my second sub in 2 days. The one before was a Gen 3 but made a static electric noise when powered, which was obviously not normal. Ah well, maybe 3rd time is a charm. Just ordered another Sub 4 from a different retailer coming tomorrow so will update what that’s like :) 

UPDATE: Got a new Sub 4 from a different retailer and it exhibits the same thump after 10 seconds from pausing. So, unless I’m very very unlucky, I’m inclined to say that this is normal behaviour for Sub 4 and nothing to worry about. It’s a shame that Sonos support didn’t know this or feel the same way, as I wouldn’t have had to return the previous one :(