thumping sound when gas is accelerated

I just bought a 98 subaru forrester AWD and I am hearing a thumping sound only when I am on the gas. When I take my foot off the gas or go to break the noise goes away. I have had it to a garage and they hear the noise but can not duplicate the noise when it is in the garage and cant determine for sure what it is. They said it could possibly be wear on the front axil. Any help would be great. Also how long should brake pads generally last. Thanks for any help you can give.