Clicking noise - 89 Astro 4.3

1989 Astro 4.3 engine I am getting a clicking, tapping noise on idle. Doesn't get worse if you speed it up, in fact it goes away. Doesn't sound like the low growling noise of lower end problems - it is more of a metallic click. The noise goes away if you put it in gear. When driving, it will come back when you slow down or let off the gas (like when you come up for a stop sign). Hard to tell where its coming from but seems to be either the passenger side of the engine or towards the front. Anyone know what it might be? Thanks Gary