The guts of a laptop are crammed into such a small, cramped space, that the cooling fan is absolutely critical. A replacement laptop fan and heat pipe should cost well under $50, you may even get by with a generic fan replacement for a few bucks, but the job is fairly involved and differs from manufacturer to manufacturer. I don't get excited about noisy laptop fans, I had one in my Toshiba Satellite that got noisy within a year of my buying it and continued noisy for the next four years without failing. On the other hand, you don't want to wait until you get heat damage to replace the fan. If the fan gets increasingly noisy over time or starts noisy (and slow) then quiets down as it picks up speed, I'd replace it at the first opportunity. Assuming you've owned the notebook for a while, you should be familiar with how long the fan usually takes to come on and how long it runs. If the fan never comes on, unless you're working in a freezer, it's probably dead. I just did a page on troubleshooting laptop CPU overheating problems and inspecting the laptop fan for linting or failure.
