Whenever you try to search in SharePoint and you are trying to search for a number combination – e.g. you have a custom metadata field contain productid in format P123-980 – you will notice that the search will not return the results you are expecting. The reason why this happens is that SharePoint uses a couple of files containing noise words which are omitted from the search – this will remove things such as "the", "a", … – numbers are also added in there. What you need to do:
For more info check out How to customize SharePoint Portal Server 2003 by using IFilters, noise words, and thesaurus files and also take a look at how SharePoint Search works in multilinguage scenarios
You can find the reference here.