My 2004 SL with 124,000 on it started making a rattling noise that would be the same as the RPM's for about 1 to 2 minutes on a warm start. I was getting the "timing chain tick" anyway so I had new timing chains, tensioners, guides, water pump, etc. put in, to the tune of $2200.00. One of the long guides had the tip broken off, and the small guides and the large tensioner were shot. I get the car back and the "hot start rattle" is still there! I found the answer on a Murano forum where someone said the problem was noisy fuel pressure dampers. They likened the problem to something like air getting in water pipes in a house and causing that banging sound. I had both dampers replaced, and that solved the problem.

 

Good you had it resolved. There's actually a TSB on the issue, NTB07-010a.