I have a rattle in my 2003 Buick Rendezvous on take-off, but it goes away after about 45 to 50 mph. It has about 145,000 miles.

Where does it seem to be coming from?
There is a bulletin (GM Bulletin # 03-06-05-003, dated Feb '03) for a rattle coming from the exhaust system during certain conditions.
Their resolution is an improved clamp.
Let me know if your noise is from the exhaust and I'll post the entire fix.

How can you tell if it is coming from the exhaust? All I know is there is a terrible rattle on take-off.