Perhaps you bought a high-quality FM transmitter that is supposed to provide you with high-quality audio. However, no matter how good a transmitter is, several issues or reasons can explain a static sound.
The main cause for static interference is low audio source or feed. Moreover, there will always be static noise in the background when you're using an FM transmitter.
How To Fix Static Noise In My FM Transmitter
One way to counter the low audio feed issue is to significantly turn the volume up to mute any static background noise.
Another way to reduce the static noise as much as possible is to boost your FM transmitter's signal. You can boost signals by adding an extra antenna so your car radio can receive the audio being transmitted more efficiently.
If you don't mind getting handsy and have a fair knowledge of electronics, you can take apart the FM transmitter. You can replace resistors in the transmitter with copper wires by taking them apart. Most FM transmitters will have resistors installed in the circuit to limit the signal's power. The goal here is to reduce that limit. Thus, the replacement of resistors with copper wires should do nicely.
What If My FM Transmitter Is Not Causing The Static Sound?
You can test out your FM transmitter in another car to diagnose whether or not the transmitter itself is the issue. Often you can fix the static noise by boosting the signal, adding an antenna, or turning up the volume of your car stereo.
However, if none of those work and your FM transmitter works fine in another car, the problem might be with your car's stereo.
Usually, an older radio or stereo system will become weaker at receiving signals or playing audio due to the natural degradation of its components. If that is the case for you, the best thing you could do is hire a good electrician or replace your car radio or stereo system.