I'm also experiencing creaking around the lower half, both sides I think.
Removing the (very nice) silicon case makes no change.

I tried ignoring this for a while but it occurs so often during normal usage it's quite unsettling. I also notice there is some movement between glass and polycarbonate body, which I suspect may be the cause. Maybe the adhesive holding the two has come off?

I didn't have this issue with my first 64GB (Austrian batch which suffered from chronic reboot/crashes) and so have arranged an exchange.

I really didn't want to do this as I'm really liking this phone but it's difficult to have confidence in the current unit, so thought it would be better to deal with it now than when it potentially leads to a bigger problem later and have to deal with Nokia Warranty (who finally replaced my N900 with USB detached fault with another N900 after first providing me an E7).