I'm new in this forum and i was looking answer to my question on internet and found this place. Have you got answer to your problem? I have same problem, when i put 1st gear in when standing rear axle will make "pump" noise, when i press clutch pedal it sometimes makes same noise. I have repaired my rear frame problem and welded it so it can not be the problem. I my self am suspicious about rear U-Joint, as it makes little kicking noise when i turn drive shaft little bit.
Please help me as i have run out of ideas
Excuse my bad English as well!