Why are my woofers pumping?

Correction: the Atlas has a 121 ohm load (not 100) - and sounds fine.
Replacing belts - yes, new ones ordered.
I will give the spindle a drop of oil, and fasten the spindle housing when I get the right tool.
BTW, reading up on the Hifi News test record shows that people find a lot of problems with it. On the lateral resonance test the actual test tones are different from those stated. I know from before that the record's anti-skating test is not good, in my case at least.
Some years ago Michael Fremer reviewed the Kuzma 4-point with his Lyra Titan i cartridge, pronouncing it the best arm he had heard so far. But using the calculators, resonance is down to ca 7.2 hz. Even more problematic than with the SME V (ca 7.5), since the 4-point weighs a bit more. Yet it wasn't a problem, to his ears.

Hi Raul, thanks, I reread your former post, where you wrote: "even at 100hz the Atlat compliance and effective mass puts the resonat frequency in a " solid " 8hz."  I get results closer to 7hz (arm mass 10.5, screws 0.5, cart 11.6g). It all depends on what we assume is the 10hz measured compliance (supposed to be x 1.5 to x 2 of the 100hz measure - 12 in the Atlas case). I have to assume a compliance of 17 to get above 8hz. I have found no exact Lyra specification. So this is really "blind", especially if we assume - as some do - that small variations in resonant frequency play a role, at the margins at least. I get confused by all the web debate on this, "fundamentalists" who say resonance is of key importance, some who say it varies with the type of arm, player, and what not, and some who simply dont believe in it (at least not with modern fairly "medium" cart/arm combos).

Likewise, I can find no objective measure of how much the resonance increases, by adjusting the static counterweight so it does all the lifting on the SME V, compared to using the dynamic spring mechanism (but a lot of debate about how the dynamic may sound best and be more kind to the cartridge). I do see the point of Lyra including light weight alu screws (among several types). Regardless of their problems, both the Hifi record test and the calculators indicate that the arm could be lighter.

So - back to the ears? Only listening can really be the judge? Test records not correct, calculators failing to take into account important parameters  - like the arm stiffness - you name it.   

Some of you have suggested that acoustic feedback may have played a role for my "woofing" or woofer pumping.

This is relevant not least in a four speaker system, like mine, where some drivers are pointed towards the back wall  (with the record player).

I tested a bit more, playing the new 2 x 45 rpm Jefferson Airplane: Volunteers. Volume: loud. This is a very good remaster, quality pressing, low vinyl noise. I hear this on the grooves between the tracks. When the tracks / music played, I used my stetoscope on top of the different parts of the player.

The results were interesting, more varied than I would have thought. The top of the maglev feet had next to no feedback, the sound was dead. The plinth, and also the motors, had more feedback, although I did not hear clearly "wrong" or exaggerated sound. I tried to "hold on" to the plinth, firmly, as suggested in the thread above. It made no difference.

I would like to test the platter and the record clamp, but since this rotates, I can’t do it with a stetoscope. Anyone knows of a phone app to test for rumble or noise, laying on the platter?

Since this thread is getting over and finished - the woofer pumping has stopped - I will only update if I find some clue, what the problem was. So far, I have not been able to recreate it. Maybe it was caused by a troubled spindle - not quite in place - or other drive problems, as suggested by vpi and others, above. Obviously, the slight leeway between the spindle housing and the plinth, which I need to tighten, may play a role, but in practice I don't think so, since due to the 10 kg platter weight, it settles quite solid anyway, and I cannot reproduce pumping by pushing the platter (or plinth).

I note an interesting thing, checking the player, and cleaning the belts, pulleys and platter. The Hanss uses two very silent motors, not vibrating much. But when I touch the top pulleys, slightly with fingertips, the vibrations are more notable. Sometimes it seems as if the motors develop a "trouble" position with the pulleys straining and vibrating. I can reduce the vibrations just by moving them a tiny bit. This is a behavior I recognize from my VPI HW-19. The motor / pulley does not quite "stall", but a combination of energies has made it stressed. Anyone who recognizes this pattern?

@o_holter The studder you are seeing in the spindle is typical af asynchronous motors and not a malfunction.  Download and read the manual for the VPI HW-19 MK 4.  There is a paragraph that explains this.

If your woofer pumping returns do not use a filter.  You would just be covering up the problem instead of solving it.

in the meantime, enjoy the music.