I have SRAM Apex, hydraulic disc brakes on a gravel bike and everytime I go through a muddy puddle I get a grinding noise esp from the front brake for minutes after.  If it’s a ride with lots of water it’s just constant. Is something wrong what should I be doing?

Pads aren’t worn btw 

TIA

Couple of possibilities. 

1. Its normal for them to be a bit grindy in the wet. Different rotors and brake combos might be noisier than others. 
2. Calliper slightly misaligned, so part of the pad is always too close to the disc.
3. Slightly warped disc, so part of the disc is always too close to the pad.
4. Pads have some grit bedded in. Clean with IPA (not the beery type) and give a light sanding to them. Sometimes a slightly chamfered leading edge can help to. 
5. A touch too much fluid in the system. Crack the bleed valve, and lever the pads back with the ring end of a spanner, or big screwdriver, making sure to catch the DOT fluid that comes out the bleed port, then close up the port and pump the brake lever a few times till they bite again.   

Might that not just be grit 'n' grime from the puddle? The tolerances in disc brakes are close enough to cause even pretty small grains of gunk on the rotors to rub against the pads. Aside from avoiding puddles altogether, maybe ride through a cleaner one, or wait for the dirt to wear itself off. Or give a squirt or two from your water bottle, maybe

Sounds pretty normal, usually clears after a few brakings or bit of riding

It's a built in positive feedback system for gravel bikes.

It tells you that you are off road having fun and not on boring tarmac.

Care to elaborate on the downvote?