I've had a rattle coming from my car when I first start it...it happens when I accelerate past the 3K mark and disappears after the car has been running 5-10 minutes. I've taken a look under the car and I don't see any thing loose on the exhaust? I'm trying to decide if its worth spending $100 bucks to get it fixed on the extended warranty....

 

Check this link out. I had the same issue...turned out it was the separator plate between the bellhousing on the tranny and the engine block. I inserted foam rubber tape between the plate and the bellhousing to fix the issue.

https://themustangsource.com/f637/50...rattle-477417/

 

The separator plate is one possibility. Do you have the stock exhaust or have you upgraded?

I thought I had the separator plate issue a while back, but it eventually got to where the rattle just didn't stop. I finally checked out my mufflers, and they were rotted and rattling inside. I had Flowmaster American Thunders on the car, that were about 4 years old. Fortunately I still had my stock mufflers, so I was able to swap them out.

If you are still on the stock mufflers though, that's probably not it.

Good luck