Why are my woofers pumping?

Before going on, I need to tighten the center shaft (with the magnets) to the plinth on my Hanss T-30. It is fastened by four screws from the underside of the plinth. Does anyone know the type and dimension? The screws are very hard to see unless the player is placed on the side. Thats a heavy and somewhat risky job, so when I do it, I need to have the right tool. The Hanss manual does not specify the type of screw, but they have confirmed that the shaft should sit tight. ("shaft" maybe not the correct term - I mean the cylinder that the spindle sits in.)

Tighten the center shaft - meaning: the housing of the spindle.

cleeds - thank you, yes I bought the Atlas new and it shows no obvious signs of wear so far.  You are right, it may be a spindle / bearing problem. So I want to eliminate this factor. I have a friend with a T-60 who may help me fix it, soon.

Resonance - a red herring? If so, reducing effective mass on the SME V may not be a good idea (+1 to Atmasphere, and others, for setting me on this track).

Therefore I thought, OK, let me go back to a normal setup of the arm. I screwed the upper bridge back on and adjusted the counterweight back so the spring weight does the job alone, tuning it to the 1.72g recommended for the Atlas. Then I checked, using the runout on R L Jones The evening..., on the same very high volume level as before. Did I see more subsonic cone movement / pumping? NO. Slightly more energy, touching the cones? MAYBE but not much. What strikes me is that the "tone" of the vinyl noise has gone down a bit, it reaches lower in frequency. (Usually a good sign, says my audio intuition).

This test result does not rule out a "demanding" combo that can easily pick up too much lowest-low frequency. Maybe there is a herring. But its not red?

Since the problem is mainly gone, the thread should end soon. But it would be nice with a conclusion. I had woofer pumping, I changed some things in my system, and then the pumping (or most of it) went away. Why? Not clear.

Maybe the most likely reason - for now - is that I took out the platter and reinstalled it, with a little change of the ball and spindle position, making the drive work a bit more smoothly.

However my "audio intuition" is not quite satisfied with this explanation. The pumping error seems more like a yes/no proposition, as if you throw a switch - it is there, or not there - maybe mainly resonance-related (or even electric, e g could it even be influenced by static electricity?). So far I have not been able to reproduce the error. So I dont know exactly what makes the woofers start pumping. But it seems fairly clear that if it was only the drive, or player imperfections, the pumping / rumble problems would have been much more constant. 

@o_holter

I too had reduced my woofer pumping with each of the following steps:

1. Cartridge setup tweaked by my VPI dealer
2. Replaced my cartridge
3. Added a Symposium Seque ISO platform
4. Added Symposium VPI footers
5. Added a custom 2" walnut platform sitting on RollerBlock Jr’s under the Seque
6. Swapped my Quadraspire Q4 Evo rack with a Massif Audio Design rack (no obvious change)
7. I have an older Classic 3 and tried damping the arm with VPI damping oil, that didn’t help...

Each of these changes reduced the pumping (well, more or less) and a couple of times I thought my issue was resolved. I think your intuition is probably correct in that this problem might not be resolved.

PM me if the problem returns, I might be open to sending my filter out for you to try.

Todd