Wingerian

I've just joined Scoobynet hoping for some sensible advice on a vibration my 2.0XT ('54) Forester has developed. First noticed it when towing uphill at 70mph. Vibration seems to be coming from the exhaust, goes away when you drop down a gear/back off the gas. The vibration comes and goes but is starting to get worse. Deeper and more frequent - worse when the engine is under load. I've looked over the exhaust and can't see an obvious problem. Subaru dealer just gave the car its 100K mile service and told me there was nothing wrong with the car / never heard of an 'exhaust rattle problem mate'.

It has been suggested to me that it could either be the diff, loose baffles or heat shields. I'm no mechanic so if anyone has any advice it would be much appreciated.

Thanks