Re: Possible Solution
I also drive a Nissan, I encountered the same sort of thing a few times (in fact Dabbsy got the sound transcription spot-on!). It always turned out to be a door or boot not closed properly. Especially with the boot it was quiet easy to close improperly without it showing. Nevertheless, it's just a question of calibrating how hadr it needs to be closed an I didn't have any more issues after the first couple of weeks.
As to build quality, maybe too early for a definitive answer but so far everything is very solid and reliable
But I fully understand the 'how does stuff in a rental car work?' thing, has happened to me a couple of times.