I once had a problem like this, It could be the wii message board, try entering Maintenance Mode and see if it helps, If it does you should try and manage you data setting or update the console.
To Enter Maintenance mode:
1-Turn on your wii
2-Do not Press A on the health and safety screen, instead press and hold both the (+) and (-) buttons.
3-Continuing holding them until the health and safety screen switches to the wii menu.
4-If done correctly, a messsage will pop up informing you that the system is in Maintenance Mode
(Maintenance mode disables the wii message board, all other systems features remain intact.)
As I said I once had the same problem, I would turn my wii on and before I had time to even select a channel the wii would freeze (Along with an annoying buzz sound) Maintenance mode kept my wii from freezing, but I had to use it ever time I turned on my console. Eventually after a new update became available, and I updated my system the problem was removed. I do not know if an update can permanently solve your problem, but hopefully maintenance mode will help.