Rear bumper/fender may be differnet but the chassis will be the same... toyota would not have different chassis as that would be very uneconomical
The bumper may be the same too.
do you have any local breakers yards? have you googled for used parts specialists as you may find a website that specialising in sourcing used parts from breakers.. worked for me a few years back.
to be honest i have never known it in a standard mark 1 GS
when you say periods you mean it does it now and then when driving?
Exactly same noise?? is it electric pump noise?
I used to get this noise on my mk2 GS... but that was more in the engine bay.
I reckon it is brake servos priming and the sound travels through the firewall.