In all honesty mate I wouldn't worry about it. I noticed it with mine too, although mine doesn't sound as bad as your description. Never been a problem and I got used to it. There are a number of things I thought were a problem with my car after I had it a few weeks, then I read up on here and found most of them were characteristics of the car.
Don't worry sit back and enjoy the sun.
however if you think it is more serious then contact zmanalex, I'm sure he will have something to fix it.
Good luck with the warranty, I had a full Nissan warranty and it wasn't worth the paper it was written on. Everything is blamed on wear and tear.
However as the car is recently purchased from the garage definitely get them to take a look asap.