our 30 yr old wired smoke alarms have started emiting a short, sharp, jarring shreik off and on all day and night, like quick little blasts. i removed pest control plug -ins and plug -in intercom units, and that seemed to help. but then, the alarms started shrieking again. they are cleaned and do go off when using the test button. do we have a dangerous short in out electric system? thank you.

Do they have battery back-ups, and if so, when's the last time thw batteries were changed?

I was thinking the same thing: dying battery. Did you say 30 years old ?! I would reccomend putting in current technoligy

First off, your detectors are 30 years old. I'm a professional firefighter and per applicable codes and industry standards, those detectors should have been replaced 20 years ago. Put new ones in, period.
Whether they're wired into a common system or battery powered, given their age, your detectors most likely will not work when you need them the most. It may be a bit of an expense, but it is cheap insurance given the potential conseqences! Stay safe.

I have a remodeling business and agree with the builder and firefighter. They should have been replaced 20 years ago. You should install carbon monoxide / fire / smoke alarms. Many only need (1 or 2) 9 volt batteries you change twice yearly. They work when the power goes off keeping you safe from the 3 hazards.