my new spring bolts just came in the mail today! I'm gonna give it another shot this weekend and I'll take the advice of tightening each side up just a little at a time

 

if it rattles it's JDM for sure. hahaa
The underneath of my car rattles at about the same rpm range, but it doesn't bother me. I'm pretty sure that the catalytic heat shield is rattling. oh well...

 

Alright. Gave it another go with the new spring bolts to no avail. I finally got smart and used a shop light to look up inside the manifold to the threads. They were completely smooth up there lol. So I went to the local ace hardware and bought a 1/4 bolt, washer, nut and locking washer. It took a little bit of beast mode to put enough pressure on the spring for the bolt to go through enough to catch the nut on the other side. Btw the stock cat is back on there so I think I'm going to sell the test pipe. The good news is no more rattle! big thanks to everyone who supplied their advice!