Thanks for the prompt and helpful advice. I have now tried changing the seeds on the two noise blocks to different negative values (-42 and -99), and it does indeed appear to have resolved the problem. Or having one positive (+42) and one negative (-99) also solved the problem.
I think you are right that this should be in the documentation. At the moment the documentation is very misleading, in that a) The default seed in the GRC block is 42, which is a great number, but unfortunately is positive, and b) the documentation when you click on the block says "noise_source_c(gr_noise_type_t type, float ampl, long seed = 3021) -> gr_noise_source_c_sptr", which again hints at a positive seed (i.e. of