Hi and welcome to the forum.
The likeliest cause is dirt or debris in your keybed switch contacts.
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Each key has two sensor switches that close sequentially as you press the key. The Nord detects key velocity by measuring the time between the switches closing. The switches are constructed from rubber membrane domes containing a puck of carbon (see below).
Normally, when the key gets pressed, the rubber deforms, the puck contacts the carbon pads on the circuit board and this closes the switch. These rubber domes have a vent to the outside world to allow air to escape when the key is pressed. When the key is released, the dome re-inflates and sucks air back into the dome. Unfortunately, it sometimes sucks dust or debris into the key switch contact area. When this happens the one or both of the switch contacts cannot close and the Nord measures incorrect velocity.
For example, if the "first" switch gets blocked, and the second switch closes, the Nord interprets this as maximum velocity (it assumes zero time between the two contacts closing). This causes a loud key for velocity-sensitive instruments (e.g., piano).
If you feel up to it, there are guides on how to disassemble the keybed to clean this on the forum.
Some people report success blowing compressed air along the sides of the malfunctioning key. I think this requires a bit of luck - to blow the debris out and not blow more debris into the neighbouring keys.
If you want to attempt the (disassembly) repair yourself search the forum for guides. If you can't find what you're looking for, post a reply and usually someone will help locate relevant threads.
Also, if you don't have one already, I recommend a dust cover for your keyboard to help avoid future recurrence of this problem.
It must be hardware related as its the same physical key even when I shift the octaves etc.