I had a refurbished driveshaft put in about two years ago (from driveshaftspecialist.com). There is a low frequency rumble that begins right at 35mph and stops at around 40. It's more of a sound than a vibration. Worse in 4th gear, better in 3rd. Worse with acceleration, better without throttle. Motor mounts replaced within the year. Anything else to check? For a 1990 325i coupe with a 4.10 rear diff. Thanks for suggestions...