This noise is exactly noise from my 02 Nissan Altima 3.5SE V6 same engine with a 03-07 MO, this noise is come from the valves tapping. Used one bottle of a Lucas oil products heavy-duty oil stabilizer from Adavance auto Parts or Wal-Mart for your next oil change , this noise will go away after a couple hundred miles.
sounds like noisy valve lifters,
high mileage, long interval between oil changes, dirty fuel injectors, and a worn crankshaft could lead to this.
try using a higher octane gas, say 94, this might quiet down the noise.
Thank you all of you for giving me advices. I tried to change heavy-duty oil, but it doesn't solve the problem. In addition, it's not a normal sound because it just start making noise recently and it's much louder than the engine sound that I had before.