The most innocent explanation would be some water/condensation in the exhaust system. And this might be the case.
However, I hate to say it, but it sounds like the symptom of a leaky head gasket, with coolant getting into the combustion chamber. See if you have a slippery residue on your tail pipe. If it is just water, it won't be slippery, if it is coolant residue, it will be slippery.
How is your coolant level? Has it gone down at all since this started?
You can also remove and inspect the spark plugs, and see if 1 or 2 seem to be exceptionally cleaner than the others (coolant leaking into a cylinder can make the plug look very clean). Particularly if you find two adjacent plugs that are much cleaner than the other two.
A shop can test to see if there are any exhaust gases in the cooling system (a diagnosis aid), and I also believe there are some test strips which can show this, although I have never used them. An auto parts place could advise on this.
If it is under warranty, I would get it into the dealer ASAP to be sure the symptoms and complaint is entered into the formal system, so that you don't miss out on any warranty if it does turn out to be a head gasket problem.