I sew mostly with a treadle machine...so there are no warning beeps. :shock:
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.

One of my machines will stop and beep when the bobbin gets low. What's annoying about this is I usually have 2 or 3 inches left to sew, and I know there is 9 or 10 inches of thread left on the bobbin. So I hit the "Ok" button, hit the gas, get maybe 2 stitches along, then it stops and beeps again, and won't start unless I either hit "Ok" or replace the bobbin. It gets infuriating.

I don't know what kind of machine you have, but mine is a Babylock Serenade, and I can open the bobbin access door after the low bobbin indicator goes off, and that will allow me to finish off the bobbin. Try that.

Check to see if there is a tiny light on the inside of the door to the bobbin access area. Mine looks like a tiny glass bubble, and the way the low bobbin indicator works, is that light shines onto the clear bobbin, hitting thread. When the thread empties out most of the way, then beam of light can then shine through the clear bobbin onto a sensor behind the bobbin, which sets the low bobbin alarm off. If a thin film of lint covers that tiny bubble light, then it can't shine it's beam of light, and the low bobbin alert won't work. Try wiping it off with your finger, fabric, or a Q-tip regularly and see if that saves you a trip to the repair shop. Certain threads are lintier than others and cause mine not to work. Other threads are fine.

Thanks for the tip, but unfortunately that won't work. The bobbin is the drop-in kind, and I can't open it while sewing.

I have to keep an eye on my seam-----------only warning system I have. I usually figure it out in a foot or two. tee hee