The TBs are never silent.
They are quiet(er) when they are new, but as they age and break in the noise becomes more prevalent. This is due to how they work, they're little DC motors that are having to hold the TB open against a spring. This creates vibrations that become more audible as the bearings wear in.
There's seriously nothing to fix until the TBs begin to malfunction from bearing play or (more likely to be an issue first) the brushes wear out/get too dirty to function and the TB has to be serviced (replaced by the dealer, no one fixes them, they just R&R and send it off to be "recycled", but I have personally refurbed DBW TBs in my garage without issue).
Again, it's a non issue and you can only hear it on this car more than others because the TBs are front and center, not covered up by feet of equipment to muffle the sound (and there's two of them).
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