i am going to change the impeller on my 99' 25hp merc. how hard of a job is this ive never done it before.

Not difficult but I think I would wait until you get the rattle checked out.  If things turn out ok with that, have the dealer order you a factory service manual for it.  They are usually less than $40 and worth their weight in gold when it comes to doing minor maintenance like that.  You will save enough just on changing the impeller to pay for the manual.   The Clymer manuals are better than nothing but you can't beat the factory manual.  A lot of your chain autopart stores have them and most larger boat dealers have them for sale.  If all else fails, you might even find the service manual or Clymer on ebay.

what Way2Slow said. don't worry about the inpeller right now. figure out what the noise is. Changeing your inpeller right now is like trowing $$ out the window. Next time let me know so I can stand outside the window.

I had said in a past post. I would have that motor looked at. My 75HP Suzuki made noises just like you said. I just kept running it, waiting till the season was over. when The motor blew the power head, my season ended early. The only thing I think you should be worring about is figureing out what that noise is and how to fix it, before you do anything else