I have had a problem with BRK 9120B smoke detectors in one of the new houses that we just finished. Every other day or so one of the smokes would start blinking and would give a single beep. Not the low battery chirp, but a loud 1-2 second beep. Tried reseting each one as it happend. The home owner obviously got tired of this after a while, so we replaced all 7 smokes with new BRKs. They were ok for about 2 days and the whole thing started over again. Has anyone heard of this or know what might cause this?