Not the oil.

Took it to a Nissan dealer and they did a repair on the catalytic
converter which is covered under the Nissan Service Bulletin NTB08-001.
However, that did not do the job. My further research indicates the
problem may be related to that mentioned in service bulletin NTB08-014
which is outlined below.

I'll take it back to the dealer again and have him look at this as a
possibilty. I have created a .wav sound of the noise so I can let him
listen to what the actual problem sounds like when the engine is cold.

You can do a Google search on "altima ntb08-014" to see more about this
as a possible solution.

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Description: 2007 NISSAN ALTIMA; QR25 ENGINE RATTLE NOISE WHEN COLD
Rattling or knocking noise from the engine when all of the following
conditions are met:
1. The rattle noise is not coming from the catalytic converter heat
shield (refer to campaign bulletin NTB08-001), and
2. The engine oil level is in the operating range, and
3. Engine is OFF for 2 hours or more (cold soak of 2 hours or more), and
4. Cold start with engine coolant temperature below 122°F (50°C), and
5. The noise lasts for 1 minute or more during engine warm up, and
6. At idle or low speeds—engine rpm below 2200.