Shut it down and get the engineers, that is the best advice.
Some years ago now one of our brave crews reported feeling a thud at the rear and a slight increase in vibration, they landed on, jumped out and had a quick look around the back with rotors running then feeling happy flew back to base.
When we removed the tail drive shaft covers a number of drive shaft support bearing mounts had failed, a closer inspection of the tail rotor found evidence of a heavy plastic bag (agricultural type) having passed through it.