My first reaction was maybe that this was a scan from a print on a matt or textured paper and that this was picking up a "grainy" pattern from the paper surface, which shows up more significantly when the scan is enlarged on the PC screen.
If it's not this, you really need to examine the neg with a strong magnifier or low-power microscope to exclude something in the printing. I've used FP4+ and HP4+ quite a lot recently, and 6x4 prints are substantially grainless with the standard developers I've used. Scans of the negs viewed on screen or as inkjet prints also look OK.