A couple of months ago Son 'o Gutz started living up to his predecessor Rumblegutz's reputation. I started getting an odd vibration/chirp that was road-speed related but it would come and go. Over time, it was more noticeable as it would excite panel resonances in the dash and console. It made its presence known when the driveline was being flexed, such as in a low-speed turn or when starting from a dead stop.

Historically, I've replaced the center DS bearing as well as the flexible coupling when each has failed, and the current symptons didn't line up with prior experience. But the center bearing and support had spent over a dozen years in the AZ sun so I planned on swapping them out when I dropped the driveshaft.

I finally got under the car last weekend, and was thrilled that the U-joints were in fine shape, with no slop or joint stiffness present. I also noted that neither U-joint sported a plugged hole for a grease fitting, which runs counter to Art Benstein's experience.

The surprise and the direct cause of the odd vibration was found when I pulled off the forward DS section. One of the six M12 bolts in the flex coupling arrangement was loose - the nut had backed off maybe half an inch. With each mating side of the drive train having only 3 "fingers", the loss of one finger's worth of clamping force was enough to cause flexing whenever the driveline was other then perfectly straight - which explains the problem being more noticeable on startup and on turns.

Clearly not a common problem, but offered to help others diagnose odd driveline vibrations. The errant bolt took a pounding... pic attached.