[hellokitty]

I am fairly certain that I found the cause of the rattle even with the welded flange. The welds were fine in my case but the rattle persisted.

I took off the ARM downpipes and started hitting it and noticed a rattle. Turns out there is a lip on the inner portion of the shorter downpipe that is rattling against the flex section.

It looks like a fundamental design issue (or just a flaw in my unit) and the only solution may be to weld the inside of the downpipe.

Maybe this has been fixed in subsequent revisions but this is something you should check before installing. Just tap each downpipe and check for rattles before putting them on.