Does it get worse when turning right? Is it constant, or does it increase with your road speed? Is it a clickety kind of thump? Sounds like your CV joints are knackered. If you can see if the rubber boots that go around the CV joint is torn or greasy, or even gone, that would give you a big clue. Check in the local paper for someplace specializing in CV joint replacements, I have seen as cheap as $99 a side. If that is indeed what it is, do yourself a favor and replace both half-axles. Good luck, hope that helps.