I've developed a popping noise that sounds like it's coming from my rear axle. It only makes the noise under the initial load of acceleration in first, second and reverse and it just pops two or three times. It almost sounds like metal strain in a bushing or leaf spring, but it doesn't make the noise when putting any strain or motion on the rear suspension. Only under acceleration in lower gears; there are no new noises or vibrations under normal driving conditions. Any suggestions?