PTAC/PTHP units can be quiet. Depends upon the model. The GE 5800 is one of the nicest and is fairly quiet. My cousin turned a sunroom into a TV room and used one. They are tickled with it.

A tiny ducted system can work too. I did a 1 ton outdoor unit with tiny air handler in a kneewall with 5kw backup heat for a friend's mom-in-law room above a 3 car garage. No heat pump, hardly noticed the extra electric cost winter or summer.

A ductless mini split will work too. They are very quiet. Heating can be an issue because most don't have backup heat in them though you don't get very cold.

Actually at the link below, they have their ducted unit too which is what some high end hotels, especially Residence Inn are using.

Here is a link that might be useful: GE Zoneline