Check and see if the turbine wheel is rubbing the NGV. This can happen sometimes and cause the noise you are talking about. A piece of grass in the engine is not going to make noise. It is almost impossible for fod to get to the bearings. If it were mine or one of my customers engines, I would send it back for inspection. You take a very high risk if you choose to run it again and one of the bearings let go...........MAJOR damage usually. I am sure that if you check with Jetcat, they will want to see it.

Mickey