Most decent air conditioning manufacturers now do a domestic range of splits, these feature a virtually silent outdoor unit.

Quoted figures from the manufacturer I sell are in the order of 38/25 dBA high speed fan/low speed fan and 46 dBa for the condensing (outdoor unit).

Copper insulation and fitting £250 - £350 dependant on run/complexity etc...

All portable units have a very limted use factor....less manufacturers now sell them and it is a declining market...

The plus side of a split is that you are investing in energy effcient heating and that some (not all) now feature inverter driven compressors...