I found a toilet paper tube in my sons roomthe end was capped with a dryer sheet and capped with a rubber bandthe tube was extended with another tube, and the bottom half was stuffed with more dryer sheets
Most fire alarms actually have a life span. After so many years they will beep like that even after you replace the batteries. This means that their time is up. You just need to get new detectors. Sorry, they can be pricey if you have lots to replace. Get the same ones and you will not have to replace the wiring harnesses or base plate on the ceiling.