One of the most common reasons for older cats to howl at night is hyperthyroidism
. It is a relatively common disorder in older cats, and most cats do well when treated with medication.
If she is not hyperthyroid, then there are many other possibilities. What makes you sure she is not in pain? Cats in pain rarely limp, but express pain by hiding, acting differently, eating less, and losing weight. It would not be unusual at all for a cat this age to have arthritis, especially in the knees or back. If she is not hyperthyroid, she may respond to things like fish oil (anti oxidant and anti inflammatory medication), melatonin (anti oxidant and helps with night restlessness), arthritis treatments like Adequan (joint lubaricating injections) and glucosamine, or even pain medication.
The place to begin is a good physical exam by your veterinarian, to look for disease, pain, arthritis, or internal problems. If the exam reveals no problems, I would do a blood test to look for hyper thyroidism. Hope this helps, Rebecca