The last two batches that I've made have been 5 gallon. Something curious is happening as the wort comes up to boil. It begins a gradual rocking motion that picks up and then stays there the entire boil. It is on a wheeled stand but the wheels are locked so movement is very slight. It's almost as if it's being stirred while boiling. I do have element set to 90% during boil so it is very vigorous. It's not so much a problem as a curiosity. Do others experience this action?