Woofer pulsating with no volume , Oscillation issue?

Hello all - I just picked up some new Harbeths and as I was listening I noticed the left woofer moving kind of peculiar....I turned off my source & the woofer was actually pulsating in & out with no source volume. 

I have Manley Snappers . I switched sides (speaker cables) with no input & the problem followed to the other side....must be some kind of amp oscillation issue Im guessing...? Thanks for any troubleshooting help- 


@tommypenngotti In that case you have the two most likely candidates:

1) a tube that is making mostly low frequency noise, which would result in irregular motion in the woofer

2) a regular motion is most likely a failed filter cap, even one that is only 6 months old (we've certainly seen that enough to know it can easily happen)!