Originally Posted by Prism99

Sounds to me as if the fusible is sticking to the needle, which will certainly throw off tension erratically on stitches. Fusing melds the fusible with fibers. My bet is that your light fusing left a lot of the fusible unmelted. The needle is sticking to the fusible, and the thwack-thwack is from the fabric top and backing sort of peeling the fusible off the needle as it goes up and down.