Any ideas on this noise?
 

I have been hesitant to leave this post because it is so hard to describe a noise. But hopefully someone can help me out.
I don't know if this has anything to do with it, but the noise started the day after having my 1984 1911 smogged.
About 30 to 60 seconds after starting the car I hear a thump-thump thump-thump inside the car. When it first happened I thought it was a short in the stereo causing some weird noise in the sub woofer. That was not it. It lasts for about one to two minutes then stops. The noise is not as noticeable outside of the car. But with my head under the rear of the car I can tell it is coming from the right passenger side.
Any ideas?

Anyone?

Sometimes people speak of hearing the oil return lines making gurgling noises...not sure about a thumping sound.

Is it from the return line area?

It would help if you could be more specific. Is the car rolling or stopped? Is the clutch engaged or disengaged? Is the engine under throttle or idling? Is the sound coming from the engine compartment, near the transaxle and CV joints, or somewhere up front?

When I hear the noise the car is not moving. Not under throttle just idling. Engaging the clutch does not effect the noise. The noise is coming from the rear passenger side. Near the front of the engine. Or maybe behind the rear wheel pass. side. What is strange is the noise is much more obvious inside the car rather than outside with my head jambed under the rear of the car. I though maybe the exhaust was loose banging around. I don't think that is it.
The noise really does not even seem to be affected when I give it throttle. It is very hard to pin point the sound because it only lasts about 2 minutes. Soon as it warms up it stops. And will not do it again until bone cold. I becoming concerned because it seems to be getting louder.

Interesting! Can you relate the speed of the "Thump" to the speed of the engine? or is it independent?

It seems to be independent.

Have you ever heard lifter noise? Can you rule that out? When was the last valve lash adjustment?