I had a similar issue and found through various Forester news groups
that the problem could be the wheel bearings (common problem with 98-00
Foresters). I had the Subaru dealer replace them and this fixed the
problem. Just my experience but a place to start...
JonathanMichaelCosgrove@gmail.com wrote:
> Well here is all the background and I hope to God one of you may be
> able to help diagnose:
>
> Well I set off to move from Oregon to Texas for school, I filled my car
> with an tremendous amount of odds and ends and drove off. About 1,500
> miles into the trip the thumping sound appeared. For the last 500 or so
> miles the car has made a distinct thumping sound. It appears the sound
> occurs most frequently when accelerating from a stop and seems to
> dissappear when reaching full speed. This sound doesnt seem to occur
> when accelerating from a stop in reverse though? The sound appears to
> be comming from the right rear wheel region. Trying to diagnose the
> problem myself I have noticed that the sound is much louder inside the
> car than when standing next to the car while someone else drives
> it..although it is still audible that way too. Please any ideas?