How does the sound level differ between Buck-Boost and isolation transformers?

Buck-Boost transformers, connected as autotransformers, will be quieter than an equivalent isolation transformer that is rated for the same load. The isolation transformer would have to be physically larger than the buck-boost transformer, and smaller transformers are quieter than larger ones. For example, a 10kVA is 35dB and a 75kVA is 50dB.