what can it be? remove the rca's to the other side of the car?

Yep, don't run the RCA's along side the power cables.

If you move them and still get noise then fit a ground loop suppressor.
I used this one - http://caraudiosecurity.com/shop/produc ... /2757.html

would be best to cure the problem rather than just suppress it

Make sure ya remote cable from head unit is also not next to ya rca's

you should never run the audio and power cabels down the same side. i was told it can cause a fier.

as the others said it is a must to run the power cables seperate to the rca's but i had this problem and i had run the cables completely seperate, i found it to be the earth connection, our cars are one of the worst i've ever dealt with to get a decent earth connection. it's kinda trial and error to find the best place, i tried lots and just couldn't cure it, think in the end i found the bolt that holds the rear seat to it's bracket to be the best place. give it a go but def the earth is a major cause of the problem you mentioned.