Sounds like the lubricants used in the lens has dried out, if you have the machanical knowledge and tools, it can be done but if not get it sevice by someone who knows what they are doing.
This normally only happens with very old or damaged lenses, unless of course you have recently taken it to the beach, a little bit of sand can cause a lot of problems.
In that case I doubt that it has dried up. May be worth taking off the rubber grip, usually you can see the guide slot for the zoom underneath, this will look like a helical slot in which ride 2 or 3 screws mounted on little pillars, check that none of these screws have come loose. Remove the grip by rolling it much the same way as you remove a sock.
I just got a similar lens, 17 - 50mm with the same issues, everything works fine except it will not zoom.I am putting it off till I have spare time.
This lens was given to me and after watching youtube I feel cofident in it's repair, if I fail no lose as it did not cost me anything but a bruised go
My version is the Tamron SP 17 - 50mm 2.8 DI 2 This is one of my Nikon lenses, except for the inability to turn the zoom ring, it is stuck on 17mm. The lock button turns on and off but it is stuck. Other than that it works fine. It works on all 4 Nikon cameras, D70, D100, D200 and my D300. While we are on the subject on non Canon gear, I have just dipped my toe into the Micro Four Thirds pond. I picked up a Olympus E420 and 2 lenses, I will see if it works soon when the battery charger arrives, as it did not have one. If it works ok, then I can use it as a loaner for the students in my photography class. Some of the ladies have small hands so the size should suit them.
