The car starts fine and idles okay. Once you shift the gear to R or D, the idle drops to a point as if the engine is ready to stall. You would hear some rough clanking sound under the hood as if there is no oil -- oil level okay. Shift it back to Park and idle goes back up. The car has about 60k miles. Any guidance is deeply appreciated.

 

Can you tell where the clanking noise is coming from? Make sure the engine is not moving excessively from a broken motor mount. They are kind of under the exhaust manifolds but you should be able to see them with a light while someone is shifting into and out of gear. Do you also get a significant idle drop in park with all electrical accessories on (ie, A/C, heater fan on high, headlight on, etc)? If so then you may have an idle air control valve not operating.

 

Without knowing more about the engine noise, which the ISC can make a tapping/clikcing noise but not a load one, I would guess you have a bad ISC ( Idle SPeed Controler) otherwise known as the idle air control valve.