Strange-sounding interference

Just recently upgraded my system to a Pioneer DEH-P77MP - cracking headunit
(apart from the bright display).

My problem is that I am getting interference on the system, the like of
which I've not heard before.

Car is a Merc C-class W202 model (2000). Have 2 amps in the boot next to
the battery - one in bridged mono for the sub, and the other using both
channels for the fronts. So I've got 2 sets of RCA cables going from the
headunit to the boot and a set of speaker cables going from the boot to the
speakers. All cables are routed down the O/S of the car and then under the
driver's footwell to finally end up behind the headunit, apart from the amp
power cables which are very short since the amps are about 6 inches away
from the battery (there is a large painted-metal strut in between them).
The battery and amps are also on the O/S.

On my previous headunit I had all the cables routed down the N/S of the car
thinking that because the battery was on the O/S, it's dirty thick cable
would be going that way to the engine...... wrong! Lots of alternator whine
etc. So I figured I would re-route the cables when I upgraded and get rid
of that familiar-sounding interference - which worked well.

So now I have some new interference. It doesn't whine at all, it's more of
a hiss. It's fairly constant in volume and frequency, regardless of engine
speed - the only time it changes, in fact disappears altogether, is when I
take my foot of the accelerator. It goes silent momentarily then when I
start to squeeze the accelerator again it comes back but initially with a
momentary increase in volume (same frequency) then goes back to its normal
level. I can't hear it at higher audio volumes, but its annoying during
quiet passages. In fact it sound like I'm listening to a cassette tape
without Dolby noise reduction!

Anyone recognise this type of interference and have any suggestions for
elimination? Clearly the best solution is to locate the cables away from
the source of interference, but at the moment I'm not sure where that source
is.

A friend of mine thought it might be the fuel pump (the tank filler is on
the O/S), but I thought this would be more likely to sound like whine in
line with engine speed, rather than constant-frequency hiss.

Thanks,
Pete.