Here's the solution to this problem...

Remove the rollers and run the Roomba and see if the problem goes away. If so, here is the solution to the problem...

Each roller has a little yellow cap on one end. Pull this cap off (it should come off easily) and remove the crud that is wrapped around the "axle" of the rollers. The crud keeps the cap from going far enough down on the axle, essentially making the roller longer. This causes the roller to get into a bind and results in the roll-thump-roll-thump-roll problem. Once the crud is removed, replace the cap, replace the roller (notice how it drops in easier now?), and enjoy your Roomba again.

I thought my brushes were clean, but here is what I discovered, and it fixed my Roomba.

The yellow bearing on the opposite end the brush rollers from the gearbox will pull off the shaft. These bearings spun freely and looked clean, but when I pulled them off there was hair and junk between the end of the brush and the back of the bearing. When installed in the Roomba this gook warms up and gets sticky causing friction and stopping the brush from spinning.

Use a toothpick to pull this stuff out and the brushes spin free again. I was very frustrated, and this fixed it, works like new again.

I had a similar problem and I my case it required a bit more maintenance: I removed the brush rollers, unscrewed the grey shield covering most of the bottom of the device (4 relatively large phillips screws) and removed it, unscrewed the part that holds the brush rollers (4 medium sized phillips screws) and removed it (wiring is not a problem) before I eventually unfastened the two "arms" in each side of the brush roller cage so the blue and red part could be separated.

This gave me access to the red gearbox (right side of the cage) and allowed me to remove the cover (4 small phillips screws) and clean out the gearbox. In my case it was completely stuffed with doghair and I really wonder how it kept going for so long.

After cleaning the gearbox I fastened the red gearbox cover, snapped the brush roller cage into place again, fastened the blue mounting, fastened the bottom shield, mounted the brush rollers and put the Roomba back on its wheels. Worked like a charm. :)

The procedure takes a little time but is pretty straightforward; I really have to give the Roomba designers props for creating a device that is easily serviceable, even by amateurs, just as long as you use your eyes and head and don't rush it.

Had the exact same problem...thumping and all. Found the solution.

Once you have confirmed that everything is clean

Take out the brush and rubber paddle. At the end of both is a yellow collar that has slots that fit over blue tabs. Not all the yellow slots or the blue tabs are the same size. Match up the large slot with the large tab. This will allow the collar to snap all the way down against the brush (or paddle).

Otherwise this collar will sit 1/16" or so away from the end of the brush. This puts a slight bind on the brush / paddle that the sensors read as a full brush. (event though the brushes might be able to be turned with your hand)

Once the yellow collar is snapped into the correct position, reinstall the brush / paddle into the unit and away you go.

Good Luck.

My Roomba 530:
Side brush failed to spin and motor sounded slow.

Solution: Just like when you put your hand on the back of a hairdrier - the same for the Roomba.
Clean the filter!!! - Once you have done that - back to Roombaland!!!

Hope this helps