Transmission clunks when put in reverse

Hey guys. So my car is 1000km before the warranty ends and I have this issue. When I start my car (2006 Altima) and place it in reverse, it makes this clunking noise. If it's cold outside like -20 degree Celsius (-4 Fahrenheit), the tranny will take a second and then clunk loudly, enough to make me think the tranny is going to fall out. If I drive it around, and do the same thing, it's fine... Only when it's first started

I took it to the dealership and they said it was normal operation. I was wondering if anyone else experiences this problem. My girlfriends parents have one (2005 Altima) and you wouldn't even know it went into gear. I told that to the dealership and they said that all cars are different.

I'm not low on tranny fluid if anyone thinks that. Any ideas that I could bring up to them? Thanks for the help.

Eric