Pacifica knock question
 

I just bought an 05 Pacifica 3.5L. I have noticed a slight knocking type sound after it is warm, but only in park or neutral. You can hear it if you listen by the dash from the inside as well. It runs great other then that. Oil is full. Can anyone give me a clue here? Thanks guys... A mechanic is trying to say it is the timing belt, but it doesn't knock with rev.