Competence to do a noise assessment
The Regs say, in that always-helpful health and safety way, that someone who is doing a noise assessment should be 'competent' to do so. Which is about as helpful as a chocolate radiator as it doesn't actually go as far as defining what is a suitable level of competence to do a noise assessment. Is it a degree in acoustics, or an ability to put your trousers on the right way around without help in a morning?
Doing the noise assessment yourself
As a guide, if you are in a reasonably complex site or are going to do a lot of noise assessments I'd recommend something like the Certificate of Competence in Workplace Noise Assessment offered by the Institute of Acoustics (IOA). This is a week-long course of pain and torture, followed a couple of weeks later by an exam, but it is excellent.
IOSH also do a similar course with the snappy title of Noise at Work - Assessment and Management.
There are a lot of one or two day courses out there in noise assessment. If you have a fairly simple site then they may be suitable for you but noise is a complex subject so I would always recommend one of the two noise courses above, (and of those two, the IOA one) if you are going to dive into it and that you steer away from these shorter ones.
Using a consultant or company to do it
If you are getting a noise geek in to do the noise assessment for you then you should be asking to see their certificate of competence from either the IOA or IOSH as detailed above. If they have done the NEBOSH Diploma or some other general health and safety qualification and offer that as proof of competence then I would suggest you walk away and go elsewhere. I have done the NEBOSH Diploma and also the IOA course and can say there is absolutely no equivalence between them in terms of the detail they go into on noise assessment and noise management.
A bit of both
You may like to consider a combined approach of getting an external provider in to do a detailed noise assessment, then doing a one or two day course yourself so you can do ongoing checks and reviews of it yourself in-house. That way you can react to changes in the workplace nice and quickly (such as new equipment coming in or being moved around) without getting a consultant in every time. You can also do an annual review of the assessment yourself and save more money, and then maybe once every few years get someone in to re-do it from scratch again to give you the ongoing framework to work against.
This always seems to me to be the most sensible way of managing your noise risks - there is no point paying out for consultants every year or two if not a lot has changed.
For this internal review, the noise elements of something like the NEBOSH Diploma are probably also sufficient for that review-only process.