1996 t-bird 4.6L 90k miles. No power,
maximum 25mph and heard a rattling
noise from under the car.
Tap on the converter with a rubber mallet. If you hear rattling, then the ceramic brick inside is damaged. Also, you can perform a backpressure test or disconnect to the cat to see if that improves power.
With that speed limit, I'd suspect that the car has put itself into Limp Mode and will remain there until the problem is fixed. There are a lot of things that can cause a car to reach only 25 mph, and a clogged exhaust is one. The rattling could be the stressed exhaust system. Go to a car parts store and have codes read whether you have warning lights on or not. Then plan repairs around that. But with all the possible causes, it might be better to just take the car to a mechanic as that's likely where you'll end up anyway.