2001 Subaru Forester with Rattling in engine

Hi.  We bought a 2001 Subaru Forester with 110,000 kms a week ago.  It looks immaculate and supposedly was well looked after.  We had it one day and then went on the highway.  A rattling sound or knocking sound happens as we start to take our foot off of the accelerator.  Hard to explain but when we take the foot right off the accelerator, the noise quits. Coasting along the highway is difficult because when we take the foot off the accelerator to coast, the noise starts up. If we are in town driving, slow speeds, the rattling seems to stop.  We rev up the engine and we can hear the rattle under the hood.  We think it is in  the engine.Had the car 2 days and this pops up.  Have no idea what it could be.  Help!!