How often are noise assessments needed?
Noise assessments aren't a one-shot process and do need to be reviewed / renewed / repeated every so often, but how often? You aren't seriously expecting the Regs to be clear on this are you and give a nice 'every year' or something?
An ongoing review
The answer is that your should review it when you have reason to believe things may have changed and possibly altered peoples' exposures to noise, so for example changes to a factory layout, introduction of new machinery, changes in how long machines are used, changes in shifts, etc.
There is also a statement that "Even if you believe there have been no changes you should review your assessment every two years".
To be clear, this statement about two years is in the guidance, not the Regulation, so it is not law It is not written in blood and it is very carefully worded to say the assessment should be reviewed, it does not say 're-done'. A review can be a systematic check of the existing assessment looking to make sure nothing has changed; the machines are the same, the people the same, the shifts the same, etc. and if all is good, document that this review has been done and that's it, all sorted. Even if you had the noise assessment itself done by an external consultant you can still do this review in-house - it is your business and you know if something has changed. You DO NOT need to pay a noise consultant to come back every two years to re-do the noise assessment for you unless you have reason to believe something has changed.
There is a difference between 'review' and 're-do', remember that.
You can probably review it in-house if you want to.
A less frequent re-do
As a recommendation from me, for sites with a general noise risk I would recommend you re-do the assessment from scratch every four to five years. This is purely because although it may look like nothing has changed, some things inevitably will. Cutting heads may be increasingly worn and worn heads make more noise than sharp ones. Bearings may be wearing in a motor. A loose panel bolt can make a dramatic change in the noise levels. None of these will be apparent on a quick check so a policy of doing it again from scratch every four or five years will act as a good fail-safe for you.
If your noise risks are particularly high risk or there are often changes to working practices or equipment, consider re-doing it from scratch every three years, but make that decision based on your assessment of your risks and whether you consider the noise risks to be high or not.
When consultants say an assessment expires
And always be VERY skeptical of any noise assessment consultant or company who tell you that you must re-do the assessment every year or every two years as that is simply untrue. And if they tell you the law says you must re-do it every two years, then definitely go find another noise consultant who is more interested in doing the noise assessment correctly rather than over-selling to you. It is either over-selling or they not competent enough to be able to distinguish between law and guidance. Either way, they don't deserve your business.
As you may guess, noise consultants telling people the law requires a new noise assessment every two years annoys me a lot. I shall go and have a lie down in a darkened room now.