Fans constantly running ay 6000rpm

Hi - I'm on a pretty ancient (mid 2007) MBP. Still runs pretty smoothly, and I've kept it on older apps to make sure it does.

But lately the fans have been playing up, running at 6000rpm whenever on the internet, then not stopping when I quit everything. I've checked all processes in Aciviy Monior, and there's nothing secretly sucking up CPU, nor is the computer particularly hot (or at least, not for this model).

Any suggestions?

Thanks!