Leaving your computer on overnight will not cause it any problems, unless of course the suggestions from previous commenters prove to be true. If your system has once before showed signs of trouble after being left on for a prolonged period of time then perhaps it is best not to do so again until you have verified and (if necessary) fixed the problem.
I am in the habit of turning my system off each night but do infrequently leave it on if I'm downloading something, and I have at least one friend who leaves his on permanently, 24/7 with no adverse effects. This is only possible with maintenance though, which only involves cleaning it out once in a while!
If a fan has become loud before then it is possible, as was said, that the bearings have gone; no point in fixing it, just buy a new one! Depending on which fan it is they are very cheap. If it is the fan inside of the power supply (the large box with the wires coming out of it inside the machine), then expect to pay out more for a new one of these as you will have to replace the entire power supply.
If it is one of the primary cooling fans in the machine (such as the one placed on top of the processor, notable as it will be seated on top of a chunky, vaned metal block) then this must be replaced as soon as possible; depending on the software you use your rendering project could be very heavy on the processor, which makes it run hotter and which in turn makes the fan work harder. If the fan is already on its last legs, then this leaves your processor at risk of overheating and, in a worst case scenario, burning out completely.
But to answer your initial question again there is no harm in leaving your computer to run all night. Just be very careful that it doesn't overheat. I would recommend downloading a small program called Core Temp (
Core Temp ) which will display the temprature of your processor in the bottom left box. Post that number here and we will be able to tell you if it is safe or not.
http://www.alcpu.com/CoreTemp/