As I mentioned, I don't have 3.7s. If I were testing them, I would find some music and/or test signal that reliably produce the problem. I would use a stethoscope to reliably hear it. I would then mount felt and test, then add UltraSuede and test. Etc. The process is slow and deliberate. There is room for other input such as internal wire rattling, etc. I saw a video that showed the driver wires fed through silicone tubing. Perhaps that's a culprit. It is interesting that more than one person has heard something similar. I don't know the answer, but I do register your experience.
