We used to have the VI, VII and VIII on our smart car and had the same issues.
The noise you are hearing is most likely caused by the exhaust being too close to the heat shield and at times touching. The stock engine mounts give the engine lots of movement and with more and more miles the rubber on the mounts softens up and allows even more movement.
Couple of things you can do...
One is doing some tweaking to the heat shield to give it plenty of clearance. It may look like you have plenty, but once the engine moves the gap narrows. The other thing you can do it take the car to a local muffler shop, tell them the problem and have them heat the exhaust pipe (they should be able to tell where it is touching by the scratches on the pipe) and tweak it a bit along with the shield.
Last thing!!! Important! Make sure the bolts are on tight and periodically check them (we used to check them every 500-1,000 miles) and make sure they are tight. This exhaust has a tendency to have some vibration that may cause the bolts to loosen a bit. A bit of thread locker may help with that or locking washers.
Hope that helps a bit.
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