Occasionally when starting up my wife's 2004 G35X (65K miles), there will be clicking sounds from the engine which remind me of the way hydraulic lifters used to sound when they were failing.

 

I had a similar issue last year. Engine is in perfect shape and one day I noticed a rather loud clicking. Oil levels were perfect and all. I was running Valvoline 5w30 synthetic in temps around 0 degrees (celcius) and when I switched to Penzoil 5w30 synthetic the very next week, it ceased to click or make any unusual sounds. A year later and I have yet to hear anything. Maybe I'm one of the rare cases, but food for thought. Good luck.