Carrying on from what Rick has said,vermin are the biggest issue with these types of A/C units(apart from installers who have no idea).
Depending on the brand,the indoor is often difficult to seal.
What we have been using lately is steel wool jammed in any holes in the outdoor including the drain hole underneath.Use silicone to hold it in.Also remove the top and side panels and coat inside with lanolin or fisholene,being careful to keep it away from and components in the unit.
Rats and mice can't get through the steel wool,and geckos,ants,etc don't like anything sticky.
Steel wool is available from Bunnings,and although not that pretty,does the job well

