If the deck is noisy, remove the cover and pull the tension off the spring loaded idler and get the belt loose, but no need to remove it yet. Turn all the spindle pulleys, idlers and the gear box pulley by hand and see if they feel smooth. Also grab each blade at one end and shake up and down to make sure it is not badly loose.
Anything that does not turn smoothly, or has a lot of slack needs to be taken care of. Bad idler pulleys need replacing. Bad spindles will need bearings. I have never rebuilt the early four bolt spindles, but I think they are the same as the later six bolt ones. To replace bearings simply:
Remove the blade
Remove the bolt on top of the spindle
Use a punch or round piece of steel and a hammer and drive the shaft down out of the pulley
Once free of the pulley, the shaft will probably fall out the bottom
Now use your punch to remove the bearings
Reverse order for assembly
It's not difficult if you're comfortable with such tools. Grasshopper parts are inexpensive as commercial mower parts go. The bearings are about $9 each and there are six of them. The sheaves are about $13 or so each.
If the deck is noisy, have you been keeping the spindles greased? If not, as an experiment, see if greasing them will make them any quieter. My mower was generally well cared for, but the previous owner had not greased the spindles and some of the fittings would not take grease, so I could not see if grease would have helped them although I would have rebuilt them anyway.
Hope this helps,
Larry