I bought an L3T to use as an FRFR with my Pod HD 500, and discovered the rattle on day one, but thought it was a glitch until it starting appearing on just about any patch that was warm and bassy @ F#. Contact with the floor makes it worse. Clearly a resonant feedback issue, and I suspect it's an issue in all the speakers, especially when used without a sub at higher SPLs.
I am not sure why it's taken so long for line6 to respond and get the testing done, but they don't have endless dedicated resources to attack the many issues that arise with their products. The complexity of their gear comes with a downside. Having owned products going back to the ax2, I've seen many things take a long time to be corrected, and sometimes they just aren't corrected at all. I keep hoping that the next evolution of a given product will be better, but in many cases, we just trade one problem for another new one.
The reason I bought a "dream rig" was the promise of good tone across a spectrum of tones. The DT25 is probably the best amp they've produced outside of the vetta2, but getting it to be useable with the POD has proven to be a very time consuming adventure. I bought the L3t because I could not get a clean fender sound that was loud enough to be heard over drums. The L3t had that volume, but it came with a rattle that was a deal breaker. Very disappointing to say the least. My attempt to get a clean fender tone at typical fender volumes is not some weird esoteric situation. I've got over 4k tied up in a system that sits while I go back to my fender. I've read every guide, followed every bit of advice offered here and elsewhere, and yet I'm unable to get a good clean tone at gig volume, and high gain tones are still fizzy.
My point is, I have been a line 6 "fanboy" since the beginning and yet the ongoing struggle to get the gear to perform as advertised has finely eroded my goodwill for their products. I have always been drawn in by the promise, but generally let down by the reality.
Additionally, to hear some posters saying that they have the problem, but that it's no big deal or "only happens when..." Is somewhat frustrating to read. I agree that Line6 will never offer refunds, but they should consider buy-backs just as an automobile manufacturer might do with a product that they cannot correct. When a guy has several grand tied up in a system and it's rattling under nominal conditions, he's gonna get a little excited. I understand the anxiety caused by this issue. I think Line 6 is taking it a little too casually. Does the new fan work or not? If it's more then the fan, and they have to redesign the internals of the speaker, I would not be holding my breath for a solution anytime soon.
My solution was to return the product as I was still in my return period. If I had kept the speaker, it would be rattling away and would have ended up being a very expensive boat anchor.
My new pair of QSC k12s are not perfect, but they don't exhibit any symptoms that make it impossible to use the product in my application. Avoiding certain notes is clearly not an option for anyone having this issue.
From my point of view the best course of action is to return the product if you can and once they have posted a solution and decide if the Line 6 product is right for you at that time.