Whistling?

Do you have any other indications that Mom may have dementia?

Does Mom wear false teeth? Do they fit well?

Is it actually 'whistling', or 'wheezing?"
Ask her doctor.

@jeannegibbs - Mom has been diagnosed with mild cognitive impairment as a result of several "mini" strokes over the past years. She sometimes has trouble finding her words and completing sentences. Her short-term memory is not good. She also wears hearing aids but has neurological indicators that she is not processing her speech properly all of the time. She still has all her own teeth. It is just an annoying, under-her-breath whistling noise without the whistle, only expelled air.

@Sendhelp - It is actually a whistling-type of sound. I ask her what is the name of the tune she is whistling and she is not even aware she is making the noise. She does not have asthma or any other respiratory ailment. She just had a checkup with her GP last week.

Ok, gotcha. To rule out every possible purely physical malfunction, my last suggestion would be to blow her nose, clean it with normal saline. Make sure the whistling is not from there. It could be dried up mucous inside, whistling as the air goes by.