In Fieldbus, is the Digital Signal Susceptible To Noise?

Yes, but not so much as an analogue signal. The fieldbus pair is balanced about ground, and (using good cable) is twisted, and shielded. All these features reduce the level of induced signals into (or out from) the bus. Unlike analogue signalling, low levels of interference have no effect whatever on the accuracy of digital transmission. Interference test criteria are specified in the IEC61158-2 standard, and suggest undetected error rates of only 1 message in 4 months, even with rather severe interference levels.