its fine i wouldn't be overly worried my boy does it when hes in bed and its only up stairs this low growl and sometimes a random bark as he looks fixed at my wall as if somethings there.... iv come to grips that its probably ghost but they dont scare me and my wife so we let him growl on.... i encourage growling and barking at things hes not comfortable with it puts my mind at ease that if somethings really does happen he'l bark and i can grab the 12 gauge :)