A 2 year old who panics from loud sounds such as sirens racing heart & fast breathing, and may be seeing more then imaginary friends. Worried?

Don't worry yet. Two year olds are very reactive, sensitive to stimuli, particularly if new and abrupt. Aversion to siren noises, etc. Is quite normal. Usually such episodes are very brief and go away very soon after the stimulus stops. If the episodes start to last longer than 30-60 minutes, or if the "imaginary friends" start to bother him/her, then i would seek the advice of a pediatrician.