Strange touchpad sensation Dell XPS 9560 - gravel like feeling under fingers

[Warren82]

On the Dell XPS 9560 the touchpad feels smooth 95% of the time but then occasionally it feels as though i have a very tiny piece of gravel under my fingers which moves around. It's driving me a bit bonkers as I know the touchpad and fingers are clean so don't know what is causing it.

Anyone else notice this or can advise on the issue?