So your car also makes loud grinding only while raining? If loud grinding all the time, that is either brakes or worn wheel bearing. So either your mechanic messed up or there is another issue with your car. Pulsating brake pedal only when wet condition might just mean you brake too hard and the vehicle is trying to make sure you don't skid out. (ABS) Do you generally brake hard and fast? If you do that's the issue. If this has happened only after the brake job was done, I suggest you take it back for immediate repair as per warrenty on parts and labor. 

New does not equal known good. Plenty of new parts come to market, even OEM parts, out of spec. The brake pedal pulsating is most likely a warped rotor. (Again, even new rotors can be out of spec. Have a good mechanic measure the rotors, before installing). It could also be that the pads aren't mounted correctly or the caliper pins are semi-frozen. 

IMHO, you need a second pair of eyes on the diagnosis and repair of the brake system.