Tinnitus is a squirrely thing, changes, doesn't change, adds noises, takes them away... it's all pretty unpredictable.
If it helps, my audiologist told me that I should fully expect these noise/pitch changes as part of the disease process. She said that the brain *wants* to hear these sounds, thinks that it's *helping* to cope with whatever damage it perceives there to be. So every time one part of the brain starts to habituate, the brain may purposely create new sounds, to fill the void. In your case this seems to be happening every six weeks, for others the schedule may be different.
The good news is that changes - additions or removals don't necessarily mean the disorder is getting more severe (though it may get more/less annoying).
The bad news is that figuring out triggers, and figuring out if you *are* getting better or worse, can be very challenging, because some changes may have nothing to do with the environment or disease process at all, and may simply be the brain trying out a new tone.
Hope that helps, at least a bit.
Matt