Shall I say: "Ich mag keine Nudeln" or "Ich mag Nudeln nicht"?
Perhaps to sound more serious... (may be a personal interpretation)
But if you use a verb as a noun, the later is always used:
ex. Ich mag (das) Bergsteigen nicht - I don't like mountain climbing (here the noun Bergsteigen comes from the verb bergsteigen - to climb a mountain)
in this case you would never say: Ich mag kein Bergsteigen.
(I mean, you could hear children speak like that, or if someone wants to sound posed defiant - but in the other cases, no)
Ich mag keine Nudeln. This is the correct grammar. The second example works if you say: Ich mag diese Nudeln nicht. (I don't like these pasta.) The word order is changing if you speak about sth. generally or sth. specific.
Generally you can say if you have a definite article you use 'nicht'.(Ist das Dein Hund? Nein, das ist nicht mein Hund) If you have an indefinite article you use 'kein/keine/keiner' (Ist das ein Auto? Nein, das ist kein Auto).
In your example the words are interchangeable. If you want to make a difference you could say 'Ich mag keine Nudeln' refers to your current bias. you don't like pasta now. 'Ich mag Nudeln nicht' refers more to the fact that you don't like pasta in general. In real life you can hear both. And because Germans are quite sloppy with that your conterpart will ask you if you only now or generally like pasta if he wants to know it.
best regards Angel