My 126 had similar sounds most apparent when tank was full.
I guess this was because I had the rear board cut to make way for speakers, and thus left some gaps for the sound to run through - just as what Merv described.
What I did was to remove the rear seat altogether and added wool insulation before installing seat, and then got someone who did interiors to measure, cut out and design a rear board cover made of wool-like fabric with added insulation. During the course of putting in this cover, we figured that this was where the noise was most apparent.
Now - I don't hear a thing - even without the stereo.
126 tailed by a 203, 129 leading the pack.