1) Yup chassis earthing is vital.

What you have is called 'storing' which is a high pitch noise due to alternator RF interference (as Rossco has mentioned).

2) Make sure Power cable and Ground cable is not laid beside speaker cables. Run Power and ground cables on one side of the car and speaker cables on the other side. EDIT: And RCA cables as Jezza has mentioned.

3) You can get the noise reducer from JAYCAR if it is due to poor quality cables.

fucking LOL @ installer saying "it's the exhaust". Is this asshat working at a store, or a mate?

... what Rossco and mao said. I'm also going to bet RCA's have been placed alongside the power run.

I agree, bad earth and or RCA and power leads next to one another. Check all the wiring for the amps

And get your money back for the install.

Lol exhaust interfering so funny : D these guys have already nailed the problem on the head never run power with RCA cables but if you need help I'm more than happy to help you switch em over to the other side if you don't know what to do. I have pulled my Wrx apart many times to do my install ; )

cheers for help and yeah they all run together

note dont go private aha

No.. dont.

As mentioned above - ive seen this countless times.

RCA cables + power Cables + probably your speaker cables all bunched up into one neat little whirring noise making clump.

I'd do this:

Put your power cable down one side of the car, your RCA cables and the small "trigger - amp on/off cable" down the middle of the car then run your speaker cables out to each four corners - avoiding that power cable.

I would then check the earth "black/brown cable" thats coming out from your AMP - make sure its at least the same size as the red power cable going into the amp. Make sure the earth/negative/black/brown/whatever cable is secured to the frame of the car - metal on metal.

Then check the negative terminal on your battery - remove any crud build up and give it a general clean up.

If this doesnt work - remove exhaust - car will die - engine noise will go away.