Help please!

I have no clue....Is it not vibrate normally then? My bass cones are shifting air like a Hitachi fan right now.

 They are making a booming noise.

Perfectly normal. Caused by warping of the record, even to an extent which isn't really visible to the eye, causing the stylus to bounce in the groove. Or it could also be rumbles from the drive motor or bearing on the turntable, but this is less likely.

Old amps used to have a high-pass rumble filter to take out this very low frequency stuff. 

I dont think it can be that, because it happens on all my records and when i change decks it still happens. Its really loud and drowns out the music.

Ah in that case.....Something wrong somewhere! Does it just happen over night?

Feedback? Could be.

Does it happen when the turntable is connected but not actually playing anything, with the amp up at the kind of playback levels where you usually experience it?

If so, it's feedback. If not, then it's presumably a fault with your amp's phono stage.
 

 No, it only happens when a record is being played. I have tried it with two amps and the same thing happens. Its very odd. And very annoying!

Then if it's not the turntables or the amps, and it doesn't happen when you are playing CDs, I'm really rather baffled. Are the turntables located near to the speakers?

I still think it may be the cartridge picking up sound from the speakers and thus creating feedback. 

Are you on Stealth mode Andrew....offline posting!