2001 Impala grinding at specific RPM

For some time now my '01 Impala (163k miles) has sometimes made a brief grinding noise at 3100 RPM before it shifts from 2nd to 3rd gear, and has recently started doing the same at 2100 RPM between 1st and 2nd gear. It doesn't make the noise every time but the problem doesn't occur when the engine is revved with the transmission in neutral, which leads me to believe it's a transmission issue that is starting to get worse. The car doesn't shudder or hesitate in its acceleration. Any advice?

if you can repeat this condition regularly I would take it down to a trans shop and have them go on a test drive with you