my 2004 altima 3.5 rattles at cold start around the intake at front of motor near the firewall.

This is a tough one to answer with so may possible variables. I'd bring it to a Nissan dealer, leave it overnight, and let them see what happens after a cold start themselves. You might have to pay a diagnostic charge, but you'll also get the answer to your question! Could be heat shield,motor mounts, or even the exhaust system.

its most likely some part of your exhaust, i wouldent be surpised if it was your catalitic converter that is loose, considering nissan did a shitty job with there exhaust its easy to bottom out and break that cat

I was so happy to see your question here. I have the same problem in the same car and my mechanic has me terrified that the timing chain is about to blow, all based on the rattle. Like yours, it is only for a few seconds (about 10-30) on startup, especially on very hot days. DO NOT take it to the dealer. Four years ago I did that on a whim. The check engine light never came on and they never opened the hood but told me a new engine was essential. Been driving the car fine ever since with a new mechanic. Can anyone tell us about this rattling problem? Is it significant?

It's most likely timing chain noise. Be sure there's sufficient oil in the engine. I've done many of them when the plastic on the guides wears away the chain starts rubbing on the metal underneath.