how to clean the MacPro fans to reduce the fan noise

I'm sure somebody will come up with a reason not to, but I always put the vacuum cleaner on reverse and blow it out.

If you open the case, you may find that the dust problem is on the boards and cards. This inhibits cooling and makes the fans run faster/louder trying to get rid of the heat.

Apple doesn't offer much advice except to warn that you turn off your machine and unplug everything from power for maintenance. Even suggests disconnecting your monitor from the Mac Pro.

A lot of people use canned air to blow it out. The warning is to let the Mac cool down first because the canned air comes out cold. I find the spray air to strong to risk around delicate components.

The danger with a vacuum cleaner is static electricity. I use a little vac with an all plastic hose. I also wear a ground strap as a static electricity safety.

I too would love to find a proven safe method.

NITROGEN its very compressed and will blow out all dust but who has nitrogen in a bottle? if you look here on forums you can see that pulling the fan housing is not that hard and then the can comes in handy

I use one of those antistatic camera puffer brushes.... loosens up al the crap, and then just use canned air to blow it all away.