Funny noise through Sub/Amp PLEASE HELP

[mk3miniturbo]

Hi there,
I have a problem with my ICE install in a MK2 golf gti  .  I have a Vibe amp powering a 12" Kenwood sub, I have put a £30 wiring kit in correctly but am getting a very stange noise through the sub.  It sounds like the sub is pulsing and making a kind of quivering noise.  The sub also makes the really strange bassline if i skip forward or back on a cd, it also makes the noise while on radio.  It does not matter what level I have the volume on the sound from the sub remains the same.  When driving slow or sat in traffic the sub puts out the same tone as my engine, which from the inside of the car makes the exhaust sound deafening. 
It plays bass OK but has this really annoying undertone all the time that makes it impossible to have your stereo on low. 

Problem is sometimes it is there and other times it is not, it makes no difference if the amp is hot or cold. I have checked and cleaned the earth wire on the Amp (although not sure if there is a right and wrong place for the earth to go.  I even went to the extent of thinking at christmas that it was the head unit.  Replaced with a shiny £300 unit and lone behold its still bloody there 

Please Help me
I will provide any information that may help solve the problem
Jody

[Dom Mk3]

a) where is the earth?
b) are the rca's and power cabling separate?

[mk3miniturbo]

How quick is that.
Yes the power lead is a seperate wire to the RCA. Although they are running down the length of the car with each other.
As for the earth my car is a 5 door so found a small area at on the inner sill near the rear wheel arch.  Have cleaned it up and thought that it would be OK.
It was fine for about 6 months but then developed this noise.  Guy at Stereo place said it cant have been the amp must be an earth or the Head unit.  So changed the head unit.

Cheers

[Dom Mk3]

so they're going down the sills together? My first try would be separating them, not too hard a job. Probably easier to move RCA's than the power lead!

Where abouts are you based?

[mk3miniturbo]

I will try that this weekend.  It is do or die, will have to try most things aslong as I can get the problems fixed.  Cant work it out.  Another thing is when the sub is humming I either turn the head unit off or take the front off and the sub makes a quick thud.  It really is bugging the life out of me.
I am based up in Sheffield

Jp

[Dom Mk3]

sub makes a quick thud? crap, I know what that is but I've forgotten... crappy crap.