I purchased a C5 Z06 with 30K miles yesterday in Dallas and drove it back to Midland, TX about 5 hours away. I noticed a vibration starting at about 65 miles per hour and staying through 85 mph, that is the fastest I drove. Don’t hold that against me!
The vibration is cyclical and peaks and wanes about every five seconds. The frequency does not seem to increase or decrease with speed. It does not matter which gear it is in or if the clutch is disengaged.
You can hear it on smooth pavement and it has a lower pitch that can be felt everywhere including the steering wheel but does not seem to be tire balance/alignment related as far as I can tell.
It has brand new tires on the front and 13-year-old rear tires on the rear with 30-40% tread left.
The rotor wear seems normal for a sports car with 30K miles.
I have read on forums where several people seem to be describing the same thing but the responders don’t add much to address the issue.
With the described symptoms I am assuming the issue stems from something down the line from the transmission.
I'm going to assume it's the 13 year old tires in the back. The compounds get hard over time, the tires are not supple enough to absorb a slight vibration that maybe an out of round wheel may be giving off.
Try getting a road force balance on both wheels.
I'm with Norm1. For passenger cars DOT says 7-8 years is the max life for a tire regardless of treadwear. You are almost 2x past this life expectancy and the rubber has likely hardened.
But also with that mileage to have tires that old very much implies they spent long periods of time sitting still which is known to create the flat spots on tires.
I have also experienced torque tube couplers that are either coming apart or have been changed to semi solid or solid as creating the rolling vibration that comes in timed waves at highway speeds as you are describing. Also, failing or imbalanced clutch components. Tho typically clutch issues seem more noticeable as RPM increases. But you really need to investigate the tires, first. That is the low hanging fruit, here.
If you wanted to remove the clutch and the torque tube from the equation for testing you can bring the car to this point of vibrations and step on the clutch and also take the car out of gear. This will release the torque tube from both ends. and also the clutch can stop spinning so fast. Does this take the vibration away immediately or does it continue until the car slows?