The Yowling of the Moggies

Our cats came from the same litter some 21 years ago. One was always very chatty but recently the sound of yowling moggies (from both of them) is driving us nuts. One of them will wonder into the bathroom (the most accoustically effective room in the house) and yowl at the shower!

Theyve both had health checks from the vet - who thinks they are doing brilliantly for their age - and advice through the internet suggests its an age or demantia related thing. But does anyone have any ideas before we break out the earplugs?