Hum can be the bane of a Part 15 transmitter installation. I've experienced hum and noise on both FM (which I primarily use) and AM.
Eliminating hum can be difficult, as there are many reasons why it exists.
Take a typical AM installation, as an example. Most AM transmitters run on 12V DC, and have poor power supplies. Replacing one of these cheap wall warts with a decent regulated 12V supply can do wonders. If you can't or don't want to do that (i.e., you're using a Talking House which supplies ground through that supply), then hum can be reduced or eliminated by using ferrite clip on beads on the power lead.
Often, however, you apply the above remedies and you still have hum. Hum can also be introduced through your audio line. Using an audio transformer, which isolates the input ground from the transmitter ground often does the trick.
Finally, hum can be generated by good old RFI, from electronic devices, AC power lines in the wall or some types of lights. Sometimes just moving the antenna, or the transmitter, will get rid of hum.
I've never really had too many issues with hum on FM until recently. Experimentation (highly recommended) revealed that this hum was being generated by several cheap power supplies on other equipment (creating RFI). All it took to totally get rid of the hum was to move the transmitter several feet.
Here's a good demo of AM hum problem and how to get rid of it....
Here's my experience with hum with FM and AM. First of all FM, My FM broadcasting was done with the Decade MS-100 mostly and this particular transmitter uses an on board power supply which is much better than what's in a cheap wall adaptor and the adaptor is A/C 16 volts not DC so it's just a step down transformer.
Some humming still was there, not bad but there, but was not a power supply issue but being induced elsewhere. I first used a ferrite on the power cord from the adaptor to the transmitter with the wire wrapped around it a few times and it reduced it 50%. But when the audio was plugged in to the transmitter from the computer and processor the hum almost was gone unless you turned the volume on the radio up full but would be not heard at normal listening volumes. Even moving the audio cables around reduced it.
Now AM is a little more of a problem. The Procaster which I use for AM and I am on AM for the forseeable future uses a DC 12 volt adaptor. The one that is supplied is no good.
It's trial and error. I had 6 different power supplies and a few were horrible and a few were not as bad. The one that worked and was properly filtered is the 12 volt adaptor that Sangean supplies with their products like the WR-15 and this was almost as good as the brick style ones that computer monitors use. So I got from Sangean a few extras for back up to have on hand and this is what I use for the Procaster. The hum is not heard at normal listening volumes and to get the best signal to noise ratio I get the on air volume is as loud as can be without any distortion in the sound. I adjust the modulation depth to almost maximum with the studio processor and do the rest with the line in adjust, compression, and the audio source volume. The Procaster allows for above 100% modulation so can handle fine on air volumes as loud or a little louder than the commercial stations and the limiting does it's job.
But with AM some of the hum as demonstrated is from the house wiring around you as the RFI from the wiring is amplitude modulated and will be picked up a little from the 3 meter antenna where FM is not affected by this.
So I have the hum problem under control.