Hey I had the same exact problem. Every damn 3rd gen integra has a rattling A-pipe. The A-Pipe is the section of the exhaust manifold that directly connects to the cat. The B-Pipe is the larger section of piping after the cat. An OEM replacement A-Pipe is around $600. Buy a header instead, save yourself $350, and gain a few horsepower. I did that and the rattle has never come back.