Mine was doing this too, and we've just fixed it!
Unplug the cleaner, and remove all the water and shampoo parts (just put them to one side for safety), then turn the cleaner on it's side so you're looking at the underside of it. Near the back, away from the brushes you'll see a few little vents and a diamond shape cover that is screwed on. Remove this cover and you'll see the bottom of the suction motor.
Around this there should be a circular rubber seal, mine had slipped out of place. It needs to be put back in place, it's kind of fiddly as it needs to be pushed in here, you may find it easier to use two flat screw drivers or some tweezers.
Then screw the cover back on and this should hopefully reslove it.
If not check all the rubber seals, if any of them have moved or perished and need repalcing then you need to put them back into the correct postition or replace them. You can buy replacement rubber seals from bissell, they're very cheap.
Laura, September 2009