If you're experiencing a small leak from under the knob of your radiator, it could be due to a faulty valve gland or packing. Over time, the gland can degrade or become loose, leading to minor leaks. Try tightening the gland nut gently to see if that stops the leak. If the issue persists, you may need to replace the gland or packing material. It's also essential to check for any corrosion or damage around the valve area that could contribute to the leak.