To help you...
This "test" is to be done with Engine Off, then re-test with engine running...
Disconnect coax, does the noise (still exist?)
Turn off the NB/ANL - does it appear? (it may be weaker but it still can be heard)
Result of conditions of the above test...
Engine Off -
No noise - proceed
If you get ambient noise (which does exist due to proximity) - power lines and ground currents from such can induce a level of hum or even hash noise - arcing further up the line in the high-tension power wires can show up like a buzz - your car acts like an antenna and transfers it to the radios' chassis.
This type of test is important to conduct, simply because poorly assembled or improperly tied off harnesses' are a major problem for newer vehicles' ECM that require constant communication with external sensors. One bounce can generate a spike in noise level that forces an error condition.
Engine On -
If you need to turn off the NB/ANL to hear it AND your COAX is disconnected - then you have some Moderate noise present. You may not be able to do too much except to check harness wiring close to the radio and ensure you're mounting the radio and chassis to a firm brace or mounting structure (like the OEM Entertainment system).
Relocate ground wire even reroute coax, to a location further away from the Dashboard to see if the noise is from a poorly shielded - localized source. You may have grounded to the same member as a sensor - so you're receiving a ground current noise from it.
May need to look into how well the vehicle is put together. Newer vehicles are harder to pinpoint noise sources but you can presume the Engine Control Module, ECM - or Body Control Units, BCU - whichever Acronym may apply, can induce this noise.
You also have CAN (Computer Area Network) communication systems - if they are noisy, this can lead to operation issues - can be caused by poor shielding or even loose terminals connecting the various sensors and subsystems together. Many of these ARE INSIDE THE PASSENGER COMPARTMENT - so you, your radio and they - are all floating RF about inside the cabin.
Even a sensor for Tilt, Gyro - Compass (Impact or Directional) sensors in close proximity to the radio can emit noise - you may not have much choice except to relocate the radio if the sensor is properly installed - they do have Bluetooth and 315MHz TPM (Tire Pressure Monitoring) subsystems as well as sensors for ride height that can add to your level of frustration of noise abatement. Each one of these subsystems have an internal Xtal driven oscillator that "pings" acknowledgement for receiving instructions or requesting communication -so poorly designed or weakly shielded sensors can emit erroneous signals the system may interpret incorrectly and actuate systems causing erratic driving problems you never expected.
If the noise is excessive (per your test) Drowns out everything -
- you will have to track down the noise source
- - use the radio (if it's small enough) with a power cable DIRECT to the battery and with a small tip Philips screwdriver, build or design an RF sniffer to help you locate these sources of noise more accurately. A simple coax of 6 feet with open shield on the screwdriver side, with the shield and center properly put together for the PL259 side (even a coax patch cord with one end cut off and the center insulator bent into a simple coil loop small enough to be used much like a metal detector "Disc" - can be waved over the engine - block - front bumper
- - just be aware of High-Temperatures, Voltages and moving parts so you don't get hurt.
- Listen for the types of noise - the unshielded part of the coax wire antenna can help localize that source of noise as well as the different types of noise.
- You may need to plug into the radios External Speaker using/put on a simple headphone like for your Smartphone, and wear it so you can hear the electrical types of noises versus the physical motions of the engines own internal noise.
GRAB YOUR OWNERS MANUAL...
Look up ACCESORIES or Appendices and see of you have something like this...
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When it comes to Safety. you are the most important asset - as well as liability.
If you can't get the noise to stop, reduced or otherwise attenuated. There may not be much you can do except get rid of the vehicle and/or install the radio in another vehicle.
I'm not trying to be funny here, I'm downright serious...
Many new cars now are exposing you to their RF as they attempt to communicate to other sub systems - this brings up the issues of RF exposure and radiation exposure levels.
CB radio in itself can't do too much, but power behind it can - as well as the HARMONICS of such power can expose you, and your passengers to excessive levels of potentially harmful RF - let alone to the Vehicles electronics, which can damage and harm you and the vehicle and unfortunately - it's not covered under warranty.
I believe the Cobra radio you have, can be operated in Positive or Negative Ground ignition systems - that means the radio electronics are INSULATED from the radio CASE (Chassis). How do you know? - you can do a simple check of this by taking the DVM and measuring OHMIC resistor resistance across the NEGATIVE lead to the RADIO CASE (Metal chassis) - if you need to use the 20K setting or higher to show ohmic reading - you have an INSULATED chassis. However - if you show dead short from Case (Chassis) to NEGATIVE lead. You don't - it's a NEGATIVE GROUND ONLY radio. That means you can only use the radio in vehicles that keep the Battery Negative to their frame ground.
To have to "fix your car" may save your life someday. At least you did a final quality control check to ensure your safety. A loose harness or bad sensor mounting can ruin your day, may even cause your car to crash. But a cheap Cobra radio with a poorly filtered power supply? I'd keep the radio and throw the car away if it keeps you alive.
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