rattle sound on center when car on "P" & "N" but when on "D" sound comes off

when we start the car there is a rattle sound under center specially when it is on PARK AND NEUTRAL. but when you put on drive rattle sound comes off. we already dismantled the engine and we are suspecting the chain tensioner is having he problem. do yo think this is correct? please help
it's almost a month now and still we didn't fix the problem