That sounds like the "you're outside of your target zone" warning beep. If you're just standing there, it's nagging you to pick up the pace.

I have this same unit, and I switched off the warning beep long ago; it was driving me nuts at every stoplight.

To do this, first enter "exercise" mode (where it shows your HR and is accumulating stats; not "measure" mode). Then, press and hold down the button at the lower left side of the unit, the one that you'd tap to switch the backlight on, the one nearest the little speaker icon. This should toggle all of the beeps: the warning beeps, the little chirps that confirm button presses, etc. Ahh, sweet silence.

Some other observations about this unit: first, the target HR zone it chose based on my user profile was way too conservative in its estimate of my max HR; you'll probably want to set the limits manually, if you have some idea what your max HR is. Sometimes I wonder if it's over-estimating calories burned, though I have no easy way to test that.

Second, the "walk test" was pretty useless to me: I had a hard time walking quickly enough to keep my HR above 120, so the A5 spent a lot of time griping at me to walk faster. I ended up almost jogging to keep it happy, and I suspect whatever formula they run on the walk test data probably doesn't mean much for the wild-looking gait I ended up with. Whatever.

Overall, I'm really happy with my A5; it helps me to pace myself. Enjoy yours!

-JAB