Beth 
2013 Nissan Altima Sedan 2.5 SL 4dr Sedan (2.5L 4cyl CVT)
It's the alternator folks.
 I bought mine in October used at a Nissan dealer with 11,000 miles.  I test drove it and it seemed fine.  Looking under the hood the battery had a lot acid deposits and the history report indicated that the battery terminals had been replaced.  The dealership said it was a faulty battery and replaced it.  After driving the car a month I noticed the rumbling at slow speeds.  Took it back to the dealer and they replaced the alternator at no charge to me.  Rumbling sound gone and car was performing so much better.  Fast forward to June of the next year I start using the AC and I started having a few intermittent incidents where the AC compressor does not come on.  I was still learning the car and thought maybe I didn't have the controls set properly.  In August it quit altogether.  I took it back to the dealer and they replaced the AC compressor, again no charge.  At this point the car is out of warranty and the Certified Used Car Warranty does not cover AC components.  The original faulty alternator had destroyed the internal workings of the compressor and it failed at 15,800 miles and it had caused the battery issue too.  Now with a new alternator and new ac compressor the car is as it should be and I love it.  It has the most comfortable front seats with fantastic lumbar support.   It handles well, get great gas mileage, is roomy, easy to get in and out of, including the back seat as the roof doesn't slope so much as other models in this range.  It has many safety features found only in high end cars. The CVT gives the car a smooth quiet ride.  The original factory installed alternator was under powered which caused the battery and AC compressor issues.  Hopefully it was replaced in time to not damage the transmission.  Nissan knows and the dealerships know it's the alternator and a good dealer will replace it and the components it damaged without charge for their customers. My dealer told me that Nissan paid for the replaced parts and labor.  Update 3/2018    Correction on my previous post.  It was the battery cables that were replaced.   Granted I am a low mileage driver, now have  24,000 miles on odometer.  Car is performing well.  There have been no new issues at all.