I've temporarily put it on a storage container on carpet. The first one I had I did the same - it was quiet for two weeks until the noise issue occurred. I held the replacement X1 in my hands and still heard the noise so there's no vibration from the surface it's on.
I had my laptop on the first time I powered up the X1. My laptop can be a bit loud so I turned it off to see how far away I could hear the noise from the replacement X1. I can hear it from 5-6 feet away.
If the noise I'm hearing is normal, or within the range of normal, I can live with it. The noise itself can't be heard when I'm gaming with headphones. I imagine I wouldn't be able to hear it when using speakers. My concern is if it isn't normal that it could lead to further issues down the line when I'm no longer within the 30 day return window. I'd then have to send it to MS for repair instead of getting an easy replacement.
I have to go back to the MS Store to pick up my modded headset puck, which I forgot to keep, so I may bring the replacement X1 in to see what a tech says.