Yeah tudscamp is dead on with the above, you can't just use any old fan in these things, they are micro controller speed operated. I saw the units that came out of my L2 pair and I was equally surprised as the tech that they failed the way some of us experienced. The replacement is certainly not superior in specs... except to say that it doesn't sound like a circular saw reenacting a scene from Aliens.
If you attempt to replace with a cpu fan or the like, you will probably be disappointed with the vibration sound from it. Worth more hassle trying to find a compatible fan when the techs at Line 6 already did the QA on this one.
Just for another result - my L2ts returned with a vibration sound imediately after having the fan swap out performed. I was ready to lose my cool pretty hard and realised it was a slightly different sound than the first. Had me and my friendly tech stumped for a while until he opened the unit another way and noticed the little cable that attaches to the stand switch seems to be exactly routed to just brush the cone of the speaker. after removing a litle of the cladding it was plain to see that there was too much slack on this cable. I taped the cable back to the stand top hat and haven't heard the sound since.
Just another weird rattling sound removed from these speakers. They sound so much better without that buzz