Recently have experienced a tinny rattling sound under the Riv particularly at low speeds and when shifting the trans. Finally got under the car last night and found the problem.
I had a Magnaflow cat from Hogan Performance installed maybe 3 years ago along with their downpipe. The cat has a heat shield tack-welded to the top side of the cat. The rear tack welds had broken, allowing the shield to vibrate against the cat body, most noticeable at low speeds. In fact it had become quite annoying. My solution was 2 LS104 hose clamps (7"dia) placed around it and moderately tightened, don't want to pull the heat shield down flat.
I know some others here have used Hogan's cats, if you experience similar symptoms, it's a quick check to see if your heat shield has developed this problem, before spending money on a shop diagnosis. Clamps from Home Depot: $2/ea. They could be a bit shorter, needed about 20" circumference to do the job.