Generally, aftermarket external fans aren't good for consoles as they can actually make things worse by obstructing the proper exhaust fans and cause worse overheating than they're suppose to prevent. Also if it's the type of external fan that draws power from the console, then they can cause no end of other problems.
It sounds like there is a definite problem with the airflow and the PS3's fans are having to work overtime to try and dissipate heat.
The thing is, if everything is working normally you should be able to feel heat from the exhaust fans (because that's what they do, they transfer the normal, unavoidable heat from inside the console).
The fact that you can't feel any heat (apart from the normal voltage warmth on your external fan) means that all the heat is trapped inside your PS3, which is probably why it's fans are working so hard.
The first thing I would do is remove the external fan from your PS3, then check to see whether it's fans are clogged with dust or obstructed in some other way.
After that try running your PS3 without the external fan and see whether it's fans quieten down and check whether you can feel hot air coming from you PS3's fans, because if the fan is still loud and/or you can't feel hot air being dissipated by the exhaust fans, then that would suggest that the airflow blockage is internal and you'd probably need to open it up give it a good dusting.
