The injectors are fairly noisy on my wife's 3.4 Pontiac Montana when you start it up in colder weather. I'd bet that what you're hearing. Check your oil often to make sure you aren't burning any. Also, look at your old oil next time you change it to make sure you don't have unusually large amounts of metal shavings, indicating excessive wear. My bet is on the injectors though.
Careful with switching to synthetic oil if your engine has some miles on it. Some people have problems with leaks after switching to synthetic oil.