Large Thumping noise

When playing my records not my cds, I am getting a very large thumpin noise like feedback. Any ideas?
That woofer excursion thump is very likely subsonic bass distortion. There are warps in vinyl records that create the stylus to pick up a LONG signal and your amp recreates it, sending the signal to the speaker. The amp controls the movement of the speaker magnet - hence the distortion and "thump" as the woofer magnet reaches it's travel limit. That is MURDER to speakers and draws LARGE current to your amp. Both damaging to equipment.
You're not dropping your tonearm on the vinyl suddently are you?
Could be the table is not isolated properly.
the interaction between the amp and the speakers causes this. many pre's used to have subsonic filter switches to eliminate this. a good match of amp/speakers should not be doing this.
I agree with Audiofeil that it may be an isolation problem. However I feel we don't have enough information. Could you describe your setup, component & physical, and at what volume levels you observe the thumping?
I have been using the exact same system for three years without any problems. I do have a tube integrated amp and at first I thought it needed new tubes but the cd player seems to work fine. If the cartridge is worn would this cause it? I am not dropping the arm onto the records.

The problem is getting worse as it happened about 20 seconds after I began to play a record. After I lifted up the arm it made another noise. I will check the cables to see if they are touching something.
>>I have been using the exact same system for three years without any problems.<<

That's important to know.

I suspect it's one of two possibilities. The tracking force has not been increased, accidentally or otherwise. The cartridge suspension is faulty.

Check your tracking force and if the problem persists, try installing another cartridge.