I sew mostly with a treadle machine...so there are no warning beeps.
The bobbin seems to rattle a bit when its getting low.
My Sapphire 875 is the only computerized machine I have has a low bobbin warning, but it doesn't always work right. If the bobbin hasn't wound evenly, it will tell me that the bobbin is low, even if I have quite a bit of thread left. I hate that!
Sharon W.