strobe flashing -can't stop it - Optima G3?

Hi

Our alarm which has been working perfectly for 5 years is doing something odd:

1. the strobe continues to flash - regardless of what I do to try and stop it. I've tried strobe tests, walk tests, user resets but it still flashes.
2. The other symptom is that the keypad tone has gone much louder than normal. Every kepress is deafeningly loud.
3. The alarm seems to arm and detect, sound etc all as normal, but the strobe continues to flash, even after a alarm trigger and reset.

Can someone help?

thanks