it seems you are suffering from generator/alternator whine. it usually comes in from the power line - 12v. it can also come in by RF because of worn spark plug cables or spark plugs that are not of the "resistor" type. Could also be a ground issue. Or, more likely, a combination of multiple issues.
there are multiple fixes you can try.
-- Put a line filter on the +12v line(s). switched and constant.
-- Use better shielded ignition cables.
-- Use resistor spark plugs.
-- Make sure all grounds are solid.
-- Re-route and/or shield your AUX cable.
-- Rub a potato on the radio and then burry it (the potato) in your neighbors back yard on an odd numbered day. (usually reserved for really tough cases!)
-- Re-route and or shield any input lines away from any power lines.
-- Change the circuit(s) from which you are tapping your 12v.
-- Check all electrical connections to be sure they are solid.
I've had mixed results battling electrical whine. Good luck.
If you are running a power amp, all of the fixes above should be applied to it, too. Especially check the routing and shielding of the interconnects.
Sometimes, noise can be picked up through speaker wires. If all else fails check that they are not running in parallel with power lines for long distances.
All of the above have/have not worked for me in the past.
Good Luck!!
When the weather warms up a bit, I'd be happy to give you a hand.