[kien]

One of the reasons why internal pumps are generally quieter is because the water dampens a lot of noise. That is unless your return pump is rubbing up against something like the tank wall or a pipe or something. A lot of internal pumps will come with cushions/foam to cover the bottom so that it isn't vibrating against the bottom of the tank glass. Same principle applies to external pumps in that the key is to reduce vibration as much as possible. Although external pumps don't have the luxury of water to aid in noise dampening, they do have the luxury of being able to be bolted securely to something.

Heat is another issue in itself, but that wasn't your question :-)