DIAGNOSIS That's almost certainly the sound of the plastic drive coupling stripping. The square peg on the motor shaft or the matching socket in the jar is wearing down and slipping, creating a grinding sound.
FIX Unplug the blender. First, inspect the jar's drive socket. Remove the blade assembly from the jar and look up into the jar's base—you'll see a square hole. Check for worn, rounded corners or plastic debris. Then, look at the drive peg on the motor base. Is the black plastic square peg damaged, chewed up, or covered in black dust? If either part is visibly damaged, that's your culprit.
IF THAT DIDN'T WORK If both the peg and socket look sharp and square, the noise might be coming from the motor's internal gears or bearings. This is less common but more serious. You'd hear the grinding even with the jar removed if you gently turned the drive peg by hand (don't force it).
HEADS UP Stop using it immediately if you hear this noise. Continuing to run it with a stripped coupling will completely destroy the connection and can damage the motor. A worn coupling on the base means the base needs service. A worn socket in the jar means you need a new jar or blade assembly.