MacBook fans are running high even under normal load My MacBook (non-pro) Aluminum is a few years old. Lately I experience the problem that my fans are running high even with moderate usage and the body really heatens up.
It did this before, but only under heavy load (videos, flash or compiling of source files). Now it seems like a regular thing.
I also have the comparison to another MacBook Pro, which is definitely not showing this behaviour, so I am quite sure that I am not imagining things.

Also one night when I closed the lid of my macbook, fans would not stop running (it seems it did not go to sleep mode).

Any Ideas what might be the reason?

The only thing I can imagine is actually to open up the body and remove the dust from there with a spray duster.
You should first determine if your computer is actually working harder due to some process(es) using a lot of CPU when they historically have not.  Open the Activity Monitor app and check your CPU idle percentage.

If your system is using a lot of CPU, the higher fan speed is likely justified.

If you are consistently showing >90% idle, while your fans are racing, you likely have a 'mechanical' heat issue (such as dust/grime on heat sinks or some air blockage.)

If it IS a dust issue, I'd expect you would have noticed the fan gradually increasing its 'unloaded system' speed over time.

There are also products that will let you monitor various sensors in your computer.  Some will also let you control your minimum fan speed.  My favorite is iStat Menus which has lots of options for what you want to watch and how you want to see it.