soft thump and driver moves...even with no signal

Anyone experience anything like this? Was listening to Spotify when I noticed a slight noise in between tracks. Saw the left driver move slightly then again maybe 10 seconds later or even less. No music was playing at that time. Turned volume down completely but still had the periodic movement. Similar to what you might see with a warped record. Shut things down for the evening. Hoping it's just some old tubes finally failing. Plan to change out power tubes tomorrow. Let me know if you've any insight into what might cause such a thing. Thanks in advance.
06eb264a e8d8 4a8b 840c d8d32093f6bcghosthouse
it happens on certain records that may rumble.
Hoping it's just some old tubes finally failing. Plan to change out power tubes tomorrow.

Based on the description this is quite possibly a bad tube in the signal chain somewhere after the volume control. Its likely not a power tube.
Thanks Atmasphere. That's helpful (look downstream of the pre-amp). In addition to a quad of 6CA7s, these amps each run a pair of 6922s + one 5687.
If this occurs fairly often, first thing to do is swap the interconnects from the preamp left for right and see if the problem moves to the other channel. If so, the problem is in the preamp. If not- its likely one of the driver tubes.
Thanks again, Atmasphere. Planning to swap pre-amp interconnects and also (but separately) amp/speaker pairings. Pre is tubed...2 6H30s and 1 EZ80.

BTW - changed out ALL the left amp tubes. Still got that movement after listening to a few tracks and once signal stopped. Apart from the fact this is unusual behavior, I'm concerned about the possibility of "an uncontrolled excursion" resulting in damage to the speaker.
FWIW - reversed the ICs between pre-amp and amps. The noise and driver movement shifted from the left to the right speaker. Reversed the two 6H30s in the pre to see if maybe one of them was going bad. After pulling and re-installing them, no more noise or unusual driver movement...not on start up nor on shut down. Will check again today to see if things remain OK.
Ha! Called it.

Its not likely that its a bad connection on the tube socket- that would sound different. Your handling of the preamp tubes has likely stopped the errant tube for a while, but my forecast is that it will be back eventually.

You swapped the preamp tubes left for right so now the problem will be back in the left channel if it is a tube.
Yes, Atmasphere, if a tube problem, expect it to recur in the opposite channel. Fortunately, the 6H30s I'm using are not the uber $ "super tubes" and I have spares.