Why is the tub making a knocking sound

On a front loader the tub is suspended by two springs. I've had an occasion when one of the springs broke, causing the tub to be misaligned with the front door and making noise when it runs. Try taking the top off and checking that those springs are both intact and in their proper locations. They're part number 204 on the diagram.

I hesitate to offer my other theory; eventually that same washer of mine started making thumping noises again, only this time it wasn't springs or dampers. What had happened was that the brace that supports the inner tub on the input shaft had broken loose, allowing the tub to wobble as it turned. You can see the support on the rear of your tub in this picture. If you look carefully on the side you can see the two bolts holding one of the three braces in place.

Replacing it would have meant a complete teardown of the entire unit; remove the inner and outer tub assembly, then separate the two halves of the outer tub before you can finally get to the inner tub. Way too much work for the age of that washer, not to mention the cost of a replacement inner tub was far more than the washer was worth.

I finally figured it out when I realized the inner tub had way too much movement when the outer tub was stationary; they should only ever move together as one unit. I hope that's not it, but that's the only other occasion I can think of that sounds similar to what you're seeing (assuming it's not a missing/misplaced/broken spring).