This thread is old, however I am writing this message entirely from my experience, and hopefully it helps others.
If you hear noises from the loft, means there is a infestation.
There are three types of infestation:
a) Resident infestation (means rats/mice live in your loft 24/7)
b) Visiting infestation (means rodents visit your property and go after getting their food/water)
c) a combination of both a & b, (yes! as tourists come to mate inhouse residents and they know when to be back!)
Firstly, no pest control company will give you exactly which category of infestation your loft/attic area is under. So this is where you need to be wiser.
Secondly, the most common reason for infestation are :
a) there is an entry point somewhere to your propertly either using pipes to/from from basement or gaps in the roof-tile joints.
b) Rats love to live in cozy warm area (they make their nesting during autumn end /winter season start), and insulation in the loft is the perfect place for that. They do multiply and spread their droppings, and tend to move their 'rooms' from one area of the loft to the other, where they haven't put droppings. So in short, the whole loft insulation can get nasty by their wee and droppings if the infestation is just 4-6 weeks old (In my case it all happened in 4 weeks holiday when we were away)
You only have two options to treat the rats, from my experience. Please read the aftermaths section later.
a) Do a pest control survey, seal the entry points first, (most likely down pipes or gaps in the roof tiles) and then KILL the rats living inside the loft by using poison OR
b) Attempt to drive the rats away using a ultrasonic sound emitters from 'specialist' pest control companies that promises removal without dead bodies. They temporarily seal the entry points. Monitor for movements and keep doing this step (by re-opening the seal if they 'sense' movements), till you are absolutely sure that all the infestations has been driven away and gone outside.
Aftermaths for each steps:
a) If you use this option A, expect to find dead rats over the loft. Also, the carcass can cause a lot of smell which is disgusting, highly unpleasant and also bring flies, maggots and other insects..It is huge effort removing dead rats and flies and nasty smell emanating from the lofts, especially if they have gone under the boarded part of the loft insulation, they will stick around for ages. For e.g UK houses has tiny loft, and I had such a nightmare dealing with flies and smell. However, this is the only option if you want to be absolutely sure that there are no more rats living in the loft. Removing dead rats and cleaning the loft insulation is the most challenging aspect here.
b) Using ultrasonic sound to drive away rats is not a proven method. Some companies 'guarantee' that they all will be gone in 2 months without any dead bodies. However, am not convinced or sure how credible that is. ultrasonic sound method and monitoring is definitely the 'corpse' free option, however, bear in mind that your loft already may have lot of mess (droppings, wee) which you need to clear anyways.
This is where it all gets tricky. Yes, a mice or rat can cause havoc.
I really hope my experience will help you make the right decision.
Whatever be the decision, unless you are NOT prepared to live with it, expect to spend a lot of money!