This kind of problem is difficult to track down, and the fact that you are using the receiver open squelch, rather than with PL (CTCSS) tones means that the you could be getting static from adjacent channels too.
First try adjusting the squelch. If that doesn't eliminate the static, try isolating the source. Also check your antenna coaxial cable to be sure that there isn't extra antenna wire that could be picking up the static or that you may have nicks or something to cause a "leaky" coax. What kind of antenna are you using, sometimes changing from a full 1/4 whip to a loaded base coil antenna might help.
When we had low band radios on Dodge vehicles in the 1990's, we had to filter the power supply to eliminate static, which was definitely attributable to the engine and transmission and occurred at certain RPM shift points. Our 100 watt high band transmitters also blew the transmission computer chip when transmitting, the result of very poor grounding in the unichassis vehicles. We also installed extra ground links between K plates to help eliminate those issues.
I would suggest that if the ground wire is not already run directly to the negative battery pole, you may want to try that, too, so it may eliminate any static that could originate in the ground.