Generally, with audio system noise on cars I look for three kinds of things. First the most common source of noise is from an alternator. For example if the alternator has a failed diode there will be a very loud audio whine associated with it.
The second most common source of problems is with grounding. For example if there is a receiver, couple to an audio amplifier as an add-on unit for louder volume or for subwoofer comma and the units are not bonded together effectively, there can be current loops and other types of problems which can cause bad audio. These problems can be hard to track down, and the fastest way to resolve them is to run a wire between each unit making sure that it has effective bonding to the housing of the unit with screws.
The third most common source of problems is load related. For example a heavy load such as a cooling fan turning on and off rapidly can make clicking noises and thumping noises in an audio system. These problems are easier to find, especially if you have some instrumentation to detect the changing in load.
And your case, I would be very suspicious of your RCA to low-impedance converter. If you have poor bonding of that unit, or there is an impedance mismatch, then that connection is more susceptible to noise, including things like what would ordinarily be minor such as a tiny bit of alternator whine which many alternators will have even if they don't have a bad diode.
To help you further I think you need to identify for us a better description of your problem, specifically how does it sound, and a better understanding of what the exact connection is and what the devices for your RCA to low-impedance converter. I am wondering comma for example if you really need that converter.
Also you might indicate what the equipment is for your player and the equipment is for your amplifier. Try to find the specification for the amplifier input and for the player output. My guess is that there is some kind of a mismatch either level wise or more likely impedance wise which is causing your problem.
So it is possible that the filtering in your player is defective...At least on output. Can you connect the output of the player to a headphone?
Also please do one more thing...Make sure things work right without the engine running but with everything connected the way you want it to be.
There when there is one additional thing that I have to bring up. That is that impedance mismatch as can make motor boating noises. It's not exactly with the engine RPM but it may vary with the voltage of the system, which varies a little bit with engine RPM. We need to make sure that your system is not motorboating. If you have the car idling and you have a certain noise comma and then you rapidly Zoom the engine, does the pitch change immediately? Or does it lag the zoom of the engine?
