Many battery powered smoke detectors will start to chirp intermittently and will marginally increase this chirping as their battery power gets too low to function. Even during this low power phase, they still have the ability to sound an alarm and detect smoke, but not for long. Do not ignore it, change the batteries immediately. Plus, most manufacturers make this chirping rather annoying to make you take action. Usually it will just take a quick 9V battery change and it will be back to functioning normally. Smoke detector batteries should be changed once per year regardless.