Our 19 year old cat started doing this about a year ago. The noise she makes sounds like someone is being murdered. She started doing this at night after we were all in bed and would sometimes come up on the landing and howl so loud it used to make your heart jump.
Now she does it after every meal as well and we think she`s calling her imaginary kittens to come and eat.
It is definately age related. We`ve had her checked at her six month health assessment and apart from her thyroid being overactive and the occasional bladder infection she is still reaonably active for her age. Oh yes, she also howls if the sun takes too long to get round to her spot on the settee. And comes upstairs in the morning and sits in the bedroom doorway howling until one of us gets up (usually me!!)