I wonder if its your output drive. Maybe a bearing is letting loose. You can do some easy checks before taking it to the dealer. I would remove the oil sump pan which comes off easily and usually to can reuse the gasket. look in the catch screens to see if you see any metal bearing shavings. its a easy job here is a couple pictures of the sump pan so you know what your dealing with.
yellow is the bolt holes 21 of them red is catch areas you might find heavier items in. Green is the screens (two of them) I was referring to with the covers in place that you have to pull to get the screens out for cleaning.
Below is a picture with the screen covers off the one on the left will have more metal in it if you rear output bearing is going. The output drive can be pulled and fixed without totally taking the engine out even though the book says to pull the engine. Rocket Scientist just did his with great results. he has some good pictures on this site.
I hope this helps because if you have to pay a Triumph tech it will be kind of pricey.
I will also lend any knowledge my limited Arse has