yep, airpumps can run a little warm sometimes. Pretty much the only way to make sure that airpumps keep from actually overheating is to use the correct airpump for the depth of the tank in question. Thats how they are rated.
For deeper tanks, you need a larger or more powerful airpump to get the air to go that deep. AS you get deeper in any body of water, the pressure increases. Thus, the need for a more powerful pump. If you try to run too small an airpump on a deepwater tank, the rubber flanges that actually push the air can rip and cause the airpump to quit working altogether.
Whenever you wind up with an airpump that you think is a little too noisy, imagine what it was like back in the day before plastic was invented. Here's a link to a cool page that has quite a few examples of vintage airpumps:
oldschool airpumpsPaul