For "barely perceptible" or "weak":
* Slightly
* Subtly
* Gently
* Dimly
* Vaguely
* Indistinctly
* Scarcely
* Barely
* Hardly
* Nearly
* Weakly
* Lightly
* Delicately
* Sliver
For "weakly" or "lacking strength":
* Feebly
* Poorly
* Dully
* Pallidly
* Uncertainly
For "not fully formed" or "unclear":
* Fuzzy
* Indefinite
* Uncertain
* Misty
* Blurry
For "with a hint of" or "slightly":
* Slightly
* Moderately
* Moderately
* A touch of
* A hint of
The best synonym will depend on the specific context in which you are using the word "faintly."