I've had this noise since day 1, only when outside temps were about 60 degrees.

its coming from engine compartment. only occurs when in drive and sounds like lifters but could be fuel injectors opening and closing. I maintain vehicle very well, do oil changes every 3k as well as early changes of all maintenance items, if that helps. mileage has gone down some as of recent that's why I was wondering about cracks in exhaust manifold giving o2 sensors false readings or 02 sensors just worn.