What kind of oil and filter do you use. Is it a noise on engine start-up or when you first put it in gear? Hold foot firmly on brake , with it in drive ,QUICKLY jab the gas pedal and release,see if you feel or hear an annoying /unusual thump . do the same in reverse , note the results. If nothing unusual with that I'll go back to my comment about the oil and filter. Cheap filters and the wrong oil can cause a nasty noise on start-up because the first thing they're doing , is filling the filter , not pressurizing and protecting the engine right away.try getting 2 FACTORY FILTERS AND WHAT brand & viscosity nissan recommends , do 2 oil changes within 500 miles of each other and see if the noise is gone or significantly improved. I say this from personal experience also. A few years ago I accidently put a cheap blue filter on my wifes' 1995 camry and forgot to change it back to a toyota filter , came out some weeks later and started it ...I thought it was ready to spin a bearing it hammered so bad ,for at least a three second count!!!! I looked under the hood realized what I had done (after I had already freaked out and priced oil pump & bearings etc. car had over 220K) ....oil level was okay...swapped the filter toyota OEM. noise gone ...THAT was over 60,000 miles ago .WOW horrendous noise ...$6.00 fix.Also chain tensioner and cam gear?