So I just bought a new bass

and it played great. I didn't really like the strings on it so I put some DR low riders on it and it plays and sounds even better... Except for the open A string. It is not a problem when I pluck it softly but if I get into it a little bit it makes this obnoxious sound and I can't figure out the problem. Did I do something wrong or is it a bad string? The strings sat around for a year + before putting them on. If anybody can give me an idea as to what to try and test to fix it that would be great. I am sort of new to bass setup and have only replaced strings about three times.