How it got there? I would suspect corrosion from interaction with the coolant or oil that was leaking through the gasket. Aluminum will definitely swell when corrosion takes root.

The concern is: if you try to remove the bump will you create a pit? Is the bump basically a big flake about to peel off and leave a big hole?

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)