I have a horrible rattle during cold start up that go'* away when it reach'* 120F or so. It sounds like its coming from underneath? Im thinkin maybe the cat? its a 01 ssei with 65k on it, with all the bells and whisles.

I have the exact same problem with my 2003 SSEi ever since I bought it with 21K miles. It has 37K and I really hear it rattle when cold..

Took a rubber malot and hit the Cat and it was OK, just above the cat is a heat shield and it rattled big time. Next oil change, when I have the car up a little, I'll investigate and fix. Long story short, get something and reach under there and bang around. See if yours is the heat shield also..