DIAGNOSIS A high-pitched mechanical noise usually points to a lack of lubrication on the chain or a problem with the drive sprocket at the tip of the bar.
FIX Stop using it immediately to prevent damage. First, check your bar and chain oil. Is the reservoir full? With the saw off, hold it over some scrap wood and look at the spot where the chain enters the housing near the bar. You should see a small oil port. Manually drip some bar oil into that port to help free up a potential clog. Then, run the saw at high speed (not touching wood) and look for oil being flung off the chain onto your scrap wood. If no oil appears, the oil pump or passage is blocked.
IF THAT DIDN'T WORK Inspect the sprocket at the nose of the bar. Make sure it turns freely by hand (with the chain off). If it's gritty, seized, or has damaged teeth, it will make awful noises. You may need to replace the bar if the sprocket is integral and damaged.
HEADS UP Never, ever run the chain without lubrication, even for a few seconds. The friction generates extreme heat almost instantly, which can weld the chain to the bar ("gall") and ruin both. That grinding noise is the sound of expensive metal-on-metal wear. Always make sure oil is flowing before you cut.