40Nm isnt tight as fook, but anyway...
Loosen and retighten once you get the wrench but check The disc's seated on the hub properly.
If it's pulsing, it's likely the disc's not seated correctly or it's warped.
Did you clean (degrease) the disc before fitting?
Easy to diagnose when it's in front of you and very easy to check run-out with a dial gauge if you have one and know how.
But I'd suggest you get the wheel off the ground, slowly rotate it and have a close look at any side to side movement there.
All the best!