Hodge's new 4.2

[Rossco]

I've been meaning to ask this for a while but I always forget. After a cold start (I think it's only during cold starts (morning)), after a little while of car being on, maybe a minute or so, maybe more I haven't timed it... I notice the engine revs up ever ever so slightly and the headlight's go a tiny bit brighter. It's like there is load on the electrical side that turns off. Could this be the glow plug's timing out ?
It's been doing this since I got the car. Nothing wrong I think, just curious what it is.

Noticed our work 79 does the same thing. It's usually as your driving off down the road after bout a minute of starting lights suddenly go a bit brighter. Guess it makes sense something to do with glow plugs.