The alternator could be underpowered, the shielding on your power wire could be too thin, you could have a bad ground location, or your RCAs may be insufficiently insulated.
Try moving the ground to see if it's any different, then try different RCAs, then the change the power line with a higher guage and/or more shielded wire.
The most common issue is the ground points, not just the amp's, but also the head unit's ground, make sure to mount both grounds to bare metal of the car with solid connections. Also make sure that if your RCA cables cross the power wire at any point, run them perpandicular to each other so the chance of inductance is eliminated. Could also be the ignition of the car, make sure the power wire is mounted a good distance away from the coil (and distributor), and make sure you use magnetically shielded spark plug and ignition coil wires. Being 50x4, with 300W RMS being the maximum RMS output, 8 guage power wire should be big enough.
thanks for the suggestions guys. Ive tried re-grounding my head unit and amp but still no joy! Ive noticed that the noise only occurs when i use all 4 channels instead of just 2.
The wierd thing is that ive also got an old sony xm444 amp which i tried and works fine. Could the amp be faulty?
The Infinity amps aren't the cream of the crop, but that's definitely odd to have happen. The Sony has *no* similar effects on the exact same set of wires?
No.... there is no interference noise when using the sony amp.
Either the infinity amp is faulty or there is some problem with a ground loop or maybe interference between the RCA's and power cables.
But that still doesn't explain why it only occurs with my infinity amp! Could it be because it is drawing more power than the sony amp thus creating a wider magnetic field which in turn could cause more interference between various wires???