Clutch chatter can also be from a worn clutch disc, particularly if the disc is worn in an eratic manner. Also an improperly adjusted pressure plate (part of the manufacturing process, not repairable in the field without replacing the clutch) can cause this. If the throw out bearing colar is worn so that the bearing does not hit the clutch fingers evenly it can cause a chatter. None of these things seem to be common on our cars though. I suspect that Terry is right, but you need to inspect things to really see.
Normally, a powertrain warranty does not cover clutch problems because the clutch is a wear item, similar to brakes. If the problem is from oil on the clutch from a rear main seal or the like as Terry explained, a powertrain warranty is very likely to cover the problem, both fixing the leak and replacing the oil damaged clutch. If it a Ford ESP policy and you have an oil leak that caused the problem you are likely covered. If it some other brand of warranty, you will need to read the contract carefully to see what could be covered.
I have a powertrain+ warranty, would this cover this problem?