Your body is rubbing on the exhaust when you are running it.

To remedy this, take a zip tie and feed it through a piece of fuel tubing just long enough to go around your exhaust (where you are experiencing the problem). If you are having a hard time feeding the zip tie through the length of fuel tubing, cut the tubing into smaller sections and run the zip tie through them separately. Attach your newly made silicone shield to your exhaust, and you're done.

Good luck.