About a month ago, went to play some vinyl and noticed a whistle from the phono pre. The volume and tone of the whistle changed based on the MC loading selection. I was able to eliminate the whistle by setting loading to custom, and fiddling with loading until the whistle disappeared (around 500 ohms).
Fast forward to today: no whistle at any loading selection setting (my optimal for sound is 100 ohms).
So, was the whistle part of break in, or can I expect the whistle to return at some point?