Are they still mating? That could be it.

I guess the main problem is that they're 15-17 years old, and obviously, very *set* in their ways. If you got babies, I'm positive you wouldn't have a noise problem, especially because babies usually bond with you immediately. I don't know how to advise you with older tiels already set in their ways, sadly. I do hope they'll learn to behave.

Maybe you could call an avian vet and ask for advice?