musty smell

Other

when I went to a tenanted flat a month ago I got a terrible terrible musty smell in the house. I know they dont open windows in winter but this is summer. I went several times after that and gor the flat painted through out while they were on holiday (with their permission). Still the smell is there although we opened the windows wide all week. They have been there for about 6 years and they have 2 small children. Its just a musty smell. has any one come across that before and do any of you know what it could be?. I am wondering if the carpet in the whole house will need changing which will be a big expense. they ofcourse did not complain. They are very nice tenants but don tkeep the property clean. they pay like clock work and are very courteous etc so I dont want to say much.