How to reduce noise levels for electric motors?

Noise and sound are a bit tricky to deal with. Generally the noise from the motor fan is "white noise" and not objectionable to the ear, except for large machines that were mentioned or as mentioned below. The objectionable noise normally comes from the slot combination used in the design. The slot combination generates a variety of frequencies, many of which can be easily calculated based on the number of slots, various combinations, slip, speed, frequency of operation, etc.

These generated frequency aren't a problem unless they excite something. Normally motor manufacturers gravitate toward "quite" slot combination when used with their set of mechanical parts used to build the motors. Rolled steel, cast iron and aluminum machines all have different challenges when designing them. Also, for economic reasons, manufacturers use the same slot combinations for as many designs as possible, 4, 6, 8, multi speeds, etc. This can result in different noise levels and frequencies in different motors.

Although we try to make the motors quite by themselves, it is a different story when connected to the application - in your case a fan/blower. And, as pointed out, load makes a difference. An otherwise well designed quite machine can be quite noisy when put on an application like a blower with sheet metal all around. And yes, even the motor fan noise can be a problem in some cases.

There are times when you need to design a specification different from the classic NEMA noise requirements. Here are things to look for.
* What frequencies are objectionable in your application? I have been through cases were I had a group of people standing around explaining what was objectionable to them. The noise meter said the motor was quite but people complained that is sounded "different", "insect buzzing", "there is a hum".
* Will a dBA type weighting (designed to weight frequencies close to the way the human ear would) be good enough to describe your needs or do you need something special for your application? (as pointed out) If the application is in a factory it is one thing, in an office another, in a hospital or clean room different yet again. I have seen some specifications that specify noise levels by frequency band. This can present a challenge for motor manufacturers and can make the motor difficult to source.
* What frequencies affect the application? This is usually overlooked until you have a problem. Modal analysis and other techniques can be used to identify resonant frequencies in assemblies. Equipment can be designed to make them less sensitive to motor vibrations.

Motor manufacturers, such as those I am involved with, work collaboratively with larger customer using sophisticated sound rooms with noise and vibration monitoring to test motors on the driven equipment (where possible) looking for the best motor design, mounting method and driven equipment redesign to meet special requirements. There are a lot of good sound measuring equipment and don't forget PC programs. I found it less expensive to use a PC and good software than to purchase a specialized piece of equipment. But B&K makes great instruments if you want portability and good quality. Oddly enough, iPhone applications are actually getting good enough for excellent first approximations.