I am not certain about the sound made when he gets excited. You should probably take him back to the paediatrician and have him checked out.
In terms of the teeth grinding or bruxism there may be an array of possible causes. Pain from an infection of the ear or from teething; if the top and bottom row of teeth are not properly aligned (malocclusion);nutritional deficiencies due to allergies or worms; and even stress and anxiety can result in teeth grinding. I would advice that you try and identify which is the most likely cause so that treatment may be more focused. Take your toddler to the dentist to assess for teething and/or malocclusion; have him checked up for an infection or nutritional deficiencies; if you have recently had another baby, moved, changed his routine, etc. then these are possibly causes for stress
All the best