Cackle Noise from radio via engine?

[Vindezal]

I have developted a cackle noise in line with the revs from the engine through my radio in the back ground. How do I solve this?
Is it:

Alternator?
Bad earth somewhere?
Loose wire-I just dont know how!!??
Aerial only got fitted 3 months ago and it was a new ish one

The CD player side is fine perfect.

[nikaso]

This happened to me ages ago, I think I solved it by double checking the connections on the head unit and making sure I had a good earth for my amp, it seemed to be ok then.

[Vindezal]

My amp is going in on friday so atm I just have head unit, so does that mean give some wires a wiggle?

[Vindezal]

WHAT COLOUR WIRE I.E GREEN OR SOMETHING IS AMONGST THE LOOM ON THE BACK OF THE HEAD UNIT TO CURE THIS FOR EARTH??????