Hi Binnur,
Before you do
that - if I were you - I'd try to replicate the fault with a Canon lens fitted - e.g. when the conditions are right, can you try the 70D a kit lens on, if you have one?
As soon as you mention Tamron and/or Sigma lenses, they may well lose interest in helping you, saying they cannot guarantee correct operation with a third party lens. WE both know the problem doesn't afflict the 1100D with those same lenses, but the cynic in me suspects they may be looking for a reason not to play ball, especially if you want it done free of charge/under warranty.
Also, if you do need to send it away, you can do so with the kit lens fitted instead of one of your favourites, which you'll still have available to shoot with the 1100D.
HTH, Dave
PS - Donald, obviously I'm sorry to hear you have another problem with your fav lens