As the title says but here are more specifics:

15KM down a muddy dirt logging road (it was pouring rain and we were told to leave or be stuck there for another day as the road could wash out, had no choice)
Flinging mud and rocks everywhere, the inside of the engine bay is caked in dirt. I went very slow, maybe 3-5KM the whole time.

Now at idle the front end (almost seems like the rad) shakes at idle, in a rhythmic or pulsing-like fashion. The RPM gauge does NOT move at all during this time. Power and everything is fine during this time. Turning AC/heat/fans on or off have no effect. And, again, the RPM gauge stays perfectly still (does not fluctuate during the shaking)

Once I start going again it seems to go away. If I park and pop the hood I can physically see the rad and stuff around the front end shaking when it does this. Shakes for 1-2 seconds, stops for 1-2 seconds, shakes for 1-2 seconds, stops... repeat... until I get into motion.

Again the engine bay is just covered in dirt. There's no leaks underneath the car or anything that I can tell. AC/Heat/everything work fine. No CEL. Please help!

 

A wash of the engine compartment fixed it all up.

edit: nope now I feel it while driving. Feels like a heartbeat. Major vibration briefly then stop. Over and over. Does not affect rpm or heat or anything but the rad is just bouncing around.

now I'm sitting idle and it's doing it very loud and very aggressively. Very much like a loud heartbeat but like its using a lot of power. The rpm doesnt change with surge and the heat/temps look fine and it drives ok