[Bobby78]

A header hitting a control arm wouldn't make a rattling noise. More of bang or scrape. How about a loose or broken shock mount. If the bushings are gone the washers will rattle. So will a bad body mount if the rubber is missing.
The exhaust is easy enough to check. Just get under there when the pipes are cold and push and pull on them and see what happens.

 

[Bobby78]

So you fixed the control arm bushings and body mounts? If not it could still be either. The body might tilt on the chassis enough in a right turn or the angle of the wheel in the right turn might pull on the bad control arm enough to quit it down a bit.

 

[Bobby78]

Look up in the body mount holes in the chassis. Are there big loose washers just floating around?

 

[Bobby78]

OK so that could be it.

But... "Okay, so I went out and checked all the body mounts and they were fairly loose but there was only rubber touching metal so I ruled that out." Isn'good!

 

[Bobby78]

Yup, just another job on a long list. I should probably trademark "I'll get around to it".

LOL! I'm pretty sure someone else beat you to it!