Check for loose bolts on the exhaust manifold. An exhaust leak can sound just like a rod knock (See Red Coupe above). Ask me how I know.
Bob B

OK, how do you know? It is a good suggestion. I have already checked them. The head studs were pulling out when I bought this car and I know that sound and it does sound like a knock. My experience with that sound is that it got louder and louder as time went on. This sound stays the same.
Thanks'
Bubba

I have had head-studs broken on a previous 75 911 and I can tell you that it does not sound like a "knock", more like a "popping" sound. And it's more notable/louder when the engine is cold.

Not sure what your induction system is, but I had a knock like sound on #3 that turned out to be a loose intake gasket on the CIS runner at the head - massive vacuum leak.
Pulled it, resurfaced the suspect CIS runner flange, installed new intake gasket....no more knock.

Any chance the sound goes away when the clutch is pushed in?

Todd

The sound doesn't go away when the clutch is depressed or when I put the car under load.
I will have a chance to have a pro Porsche mechanic listen to the car next week. I will report back with his diagnosis.
Thanks to everyone for their interest.
Bubba

You could check the video in post 4 in my earlier topic http://forums.pelicanparts.com/911-engine-rebuilding-forum/690671-ticking-noise-after-rebuild-70s-911t.html
Hopefully for you it does not sound familiar else I would just start tearing down the engine.

If it does it at idle, one thing you could do is jack the car up and feel each cylinder for a vibration/knock.
Depending how tough your fingers are you may feel it direct, or else with a small metal bar touching each cylinder. If it is internal and something like a piston hitting head, you must be able to feel it. In which case it would justify tearing the engine down.
Alan