There is a knocking sound that is almost metallic in nature and only occurs during agitation, not during spin. Here is what it sounds like: https://www.youtube.com/watch?v=wiL1WV0YEN0

This washer is about 2.5 years old and has only started this during the past few months. I think the sound is also worse during heavier loads, but I haven't tested this rigorously.

Any advice is appreciated -- thanks!