There are a few factors may cause this issue.
1, The location of the antenna
2, Unmatched antenna length and frequency
3, Interference from other equipment
4, Static interference
Let’s check what we could do to reduce the hum noise of the FM transmitter.
A, Please make sure operate the FM transmitter with the original charger and antenna
B, Do not put the sucker antenna on the ground, it needs to be installed in an open area and it's better in a 2m higher place or on the roof, please remember that put the antenna in a more open space as possible, and the higher the better.
C, If there’s no extend antenna, put the antenna few meters away from walls, metal appliances, etc. to avoid interference
D, If the problem still exists with the correct way to install the antenna, please check if the length of the antenna is matched with the frequency or not, the instruction on the user manual is just for reference, to confirm if it is in the best status, you have to check if the REF is lower enough, it should be lower than 2 cells. The lower the better. If it has more than one cell, you need to adjust the antenna to find the best length.
Note: if you need to fix the antenna after adjusting it in a suitable length, please use the Non-conductive tape.
E, Another important factor is the blank frequency to avoid interference from other equipment. You need to search for a blank frequency with a radio receiver, and then change the broadcast transmitter to the blank frequency to work.
F, You could also try to turn the power down and put a Ground wire from the antenna connector to the PA mixer board to reduce the hum noise.
Our best wishes to your countries!