Knock / Rattle noise
 

I have 2004 Mazda 3 with the 2.3. It sat up for few months. When I started driving it again,I heard a noise that sounded like I was about to throw a rod. I took it home and parked it. Two days later I started the car and there was no noise. The next Monday, I drove to work -- 30 miles one way. It briefly made the noise just before I got to work. On the way home, it made a quieter version of the noise. I thought some heat shield may be loose and rattling. I looked that evening and nothing was lose. The following day I drove it again. I stopped for gas close to work. When I started it up, I thought a rod was about to come through the pan. I decided to try to make it home. When I started the car the noise went away. About 1/2 way home, the noise came back but a lot milder. I parked it for a week and just jacked the car up so I could listen fromm underneath. There is no noise today'. What noise could be coming and going? Perhaps Timing chain? It could be coming from low on the timing chain side. Anyone ever see a timing chain tensioner intermittently malfunction?