Might be everything is nice and clean now and no gunk to muffle the noise. But as [MENTION=3283]caloso[/MENTION] mentioned, might be time to change cassettes.

I'd think that you'd only hear the noise in a few particular cogs if that is true. But if your cassette has many thousands of miles on it and you went beyond the first time you should have changed it, then maybe all the cogs are worn out.

It's also possible that something came out of adjustment. Maybe the mechanic thought something wrong and tweaked it without double checking the result.