04 Sierra clattering, smoking, heating

I have a 04 GMC Sierra pickup with about 50K miles on it. It has the 5.7L V8. The other day when I started it, a large amount of white smoke puffed out of the tail pipe but quit after a couple of minutes...no more problems that day. Next day, smoke again with a clicking (clattering) noise coming from the right side of the engine (facing car). The smoking stopped quickly and the clattering subsided after a few minutes on the road. Last couple of days...same issues with smoking and clattering at start up but now I've notice erratic readings on my engine temperature gauge. It will go up to around 240 when going slow but will usually come back down to normal at highway speed. Help!!