Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
When dealing with noise, always check two things first:
1. the solidity of your ground connections, and that they are both tightly connected, and the connectors are sparkling clean, no dirt whatever on the connectors, on either side of the connection. Grounding is essential to noise prevention.
2. the shielding on all cables. If shielding is broken, engine noise can get into the cable or into the line.
A third consideration: if the electronics in question are defective, if there is a defective capacitor that is supposed to filter out engine noise, then that could cause the problem you are describing, also.
And maybe you need a different antenna, and a replacement, new, antenna cable if your system includes an antenna cable.