Engine noise with amplifier

Hi all, thank you for taking the time to read my dilemma.

I have a 600w 4ch amp attached to a carpeted dual sub box. when I connect my 6x9's and set the amp to high pass with the engine running I have this very annoying interference like a high paced click that matches the engine tune.

the rca cables are running along the drivers side of the car, the power and REM cable are running along the passengers side. I have made sure that they do not cross or touch anywhere in the boot.

Even if I disconnect the RCA's the noise will persist.

My ground wire is roughly 30cm long and attached underneath a bolt which is cleared of paint where the wire is touching, it is also firmly in place.

The level setting of the amp will turn up or down this interference and even down to nothing, the clicking is loud enough to bother me when no music is playing.

Now, when I replace this amp with a Fusion FE-402 2ch amp and set to high pass with 6x9's connected, there is no interference.

Ok, so I think this must be an issue with the 4ch amp, maybe it has some problem filtering out these noises. Could anyone please offer some advice as to what could be done to solve this problem!

If you would like me to explain anything else in detail, please ask.

Thank you for reading.

update: if I disconnect the RCA's at the amp, noise stops.
If I disconnect them at the headunit end, the noise persists. I pulled out the RCA wiring from the car and ran it outside the car, still same problem. I am so frustrated right now!

ive seen and heard it all before, interference is a pain in the ass, from what uve said its not a bad ground through the rcas (common) and the other amp is fine so it has to be the amp, id say a dry/loose solder join inside the amp, probably where the rca sockets connect to the PCB (circuit board) so try that, re-solder them or if you have a good eye look for other obvious bad joins etc. ive fixed heaps of car amps this way

Try a different set of cables.

Some good info here which may help:
http://www.installdr.com/TechDocs/999502.asp

I spent ages trying to find the source of my problem, then just for the hell of it, swapped my expensive RCA cables for a budget set I had lying around.problem solved.