I installed a red tremor on my ST prem last week and didn't encounter any of the "woodpecker on steroids" sounds you speak of, just a nice clean vibration. Maybe the weights are coming in contact with something or something is loose. Surely CT wouldn't sign off on a product that turns a shaker motor into a knocking/ tapping motor. Maybe try it with the cover off and probe around to find the noise.

I love my Red Tremor and Polk PSW10 external sub woofer, they're the two best mods you can get for any newer pin and were purchased before my ST arrived as per many other pinsiders recommendations.