As far as I know (Nokia also) you have thresholds defined by you for the 5 IF bands. The interference on idle is measured only in UL, which means that only mobiles or external sources could be the reason (99% falty equipment or other bad guys – military etc.)
If you have any kind of hopping and you want to know which freq is interferenced you have to stop the hopping – no other way. But you could go to 15min measurements and stop it just for that time – it usually becomes obvious at once.
Hope I help.