WOW, this is a really interesting issue
I agree with Martin - sounds like you have a transmission issue, which is causing the agitator's fins to contact clothes when there is too much drag, such as in an empty tub or during spin. I can't imagine the noise that you're hearing when this happens.
Here's what I'd do -- since warranty is out, you may have an uphill battle on your hands, so prepare everything ahead of time:
1) First, get all your warranty receipt paperwork out, and organize it first to most recent. Make copies to give away and keep your originals.
2) Second, get the paperwork from the repairs or parts you bought yourself.
3) Third, and perhaps most importantly, take a video of the washer operating, especially during the time when you would normally hear the breaking fins. If you need to have the lid-switch 'rigged' so this can be done with the lid up, use a popsicle stick, a slim toothbrush handle, a screwdriver, or whatever you can find to activate the lid switch and keep it activated so you can video your machine.
4) Fourth - show us the video - you may have to post it on youtube, so people who know this machine well will see the problem. I am nearly certain someone will recognize the issue easily.
5) Last - prepare your facts, and write it all down and send Sears a registered letter stating that you had the machine repaired under warranty x number of times for the same highly unusual problem, that it was never fixed, and that it persists today. Show them your paperwork that you organized above, and show them the video, then tell them what you want. Make sure you know what you want before you start - either a satisfactory repair with an extended warranty in case the problem recurs, OR a replacement. Don't start a potential battle with Sears until you know what you want from the outcome.
If that fails to get you any results, 'suggest' to them that you're going to post the video on Youtube in regard to how Sears took your money, sold you an unsatisfactory machine that was supposedly operational from their outlet, and that they refuse to stand by it after it obviously malfunctions continually.
See what happens... I have a feeling that calm heads will prevail here.
Gordon