Plugging into shore power.... Buzzing noise

We have had a noisy transfer switch also. We noticed this especially after running the generator and switching back to shore power. Make sure there is nothing on when you plug in. No large draw right when you start like frig that will switch automatically from gas to electric, AC with setting low enough it kicks on right away, etc. When it happens, sometimes we shut the main circuit breaker off, wait a minute and flip it back on and buzz will go away. We also noticed it seems to be worse in cold weather. Just something we have learned to live with at times. Ours is random.