Hi @RomainAsuisse. welcome to the Sonos Community!
If you’ve purchased from Sonos.com it will be a brand new unit. We sell refurbished units, but they have their own page on our website.
The noise may be from what was previously playing on the Roam, however it’s hard to tell exactly what it might be. Next time the noise occurs, please submit a diagnostics and reach out to our support team who have the necessary tools to troubleshoot this with you. The diagnostics should show them where the sound came from.
I hope this helps!