Hi I'm not sure if the howling is the same as what I refer to as singing?
Ours is now 24 and totally blind but she still finds her way around ok (she knows where the fridge is).
She has been singing now for 6/7 years (I actually like it). Usually it starts when she comes in thru the cat-flap (sort of "I'm home!"). But she does occasionally sing at night.
If we speak or make any noise she stops immediately, so perhaps she just wants to know if someone is there?
Doesn't sound as though your's stops though?