Okay so I just had a mechanic install an HKS Dragger II exhaust that was brand new (I initially was going to do it myself but was persuaded otherwise) on my 2001 IS300 and am hearing what sounds like a tin can rattling behind my car between the speeds of 5 and 15 mph only while I am coasting. During the first two weeks of installment, my exhaust was fine and made no such sound. However, now every time I drive the car I can hear it and it's driving me crazy. I tried tightening the two bolts that connected the exhaust to what I believe is the midpipe, but that only fixed the problem briefly (no more than 24 hours). I then searched for any loose heat shields (although I wasn't entirely sure where they were) around the exhaust and was unable to find anything. After that, I went to auto zone and got a new exhaust flanger gasket, opened up the exhaust, and was surprised to find that one wasn't already in there. Once I put the gasket in and tightened everything up the sound managed to get worse. Could it be possible that I over tightened it? I've searched many forums and am unsure of how I should proceed. I don't really want to go back to my mechanic because the few things I asked him to do were mostly problematic, so I've decided I am going to do most of what I can by myself for this car. Does anyone have any ideas? Could the problem be a lose heat shield I was unable to find?