When I turn it on

What is 'it'?

The wall switch

A switch that is connected to a light and a fan?

Or are you saying that it happens with ALL of the wall switches in your house?

Any circuit.... bathroom lights, kitchen under cabinet LED lights, upstairs bathroom fan, master bedroom lights, front porch lights....

Do you mean that the light you just turned on dips momentarily shortly after being turned on, or that all the lights in the house do?

I'm wondering if you might have some large load in the house that's cycling on and off under automatic control, and causing everything to dim a bit every 30 seconds or so. By way of example, one possibility of many is a stalled motor that tries to start, then opens a built-in protective device, waits for some more-or-less fixed time, then tries again.

I had that problem in our 1974 construction. Turns out to be a real fire problem. The connection between your breaker (overloaded circuit) and busbar is arcing and the resistance is the dulling you detect. It gets worse, the bar gets welded away, Lots of heat etc. Seriously, you should be aware of the fire hazard. Rebalancing the circuits and changing out the box, and adding a second box fixed it. I also put in new ground rods just because. Arc fault breakers would not even work because that section is only protected by the big breaker at the meter so it can draw power pretty good.