Strange pronunciation of 非常 I listen to podcasts from Taiwan for listening practice and I think I hear the word 非常 [fēichánɡ] often pronounced like 飛揚 [fēiyánɡ] in them. Am I hearing it right or am I confusing it with some other word? If the word is indeed pronounced that way sometimes (perhaps in casual speech?), is it a common phenomenon or a regional thing?
I like your use of 飛揚 here. While it is comical or cultural to use an otherwise unrelated word to signify an elision – e.g., consider 醬子 in place of 這樣子 in Taiwanese Mandarin internet slang – if you are looking for an answer of more academic rigour, IPA should be used, not Chinese characters or pinyin. (Because after elision, what you spell may not always be orthographic.)

In Taiwanese Mandarin, Chung (2006:77) notices function words (虛詞) tend to get deemphasised via elision. 非常 is an intensifier (somewhat functional), so the following elision can be observed:
非常 fēi cháng /feɪ˥˥ ʈʂʰɑŋ˧˥/ → [feɪ˥˥ ɑŋ˧˥] or [feɪ˥˥ hɑŋ˧˥]
Notice how usually the initial of the second syllable tends to get dropped in quick speech.
Your including a voiced palatal approximant [j] between the two syllables (i.e., [feɪ˥˥ jɑŋ˧˥], which just so happens to be the pronunciation of 飛揚 fēiyáng) is permissible, perhaps serving as a 'glide' between ɪ and ɑ.

The following is an example where there are no Chinese characters or pinyin that can properly describe the elision of 今天:
今天 jīn tiān /tɕɪn˥˥ tʰjɛn˥˥/ → [tɕɪ‿ɛn˥˥]
While there is 雞 jī /tɕi˥˥/, when pronouncing i in the pinyin ji, your mouth is more closed and your tongue is more front than when pronouncing i in jin. This subtlety can only be reflected using IPA. Whereas obviously there is no /ɛn/ as a syllable in Mandarin.


References
Chung, Karen Steffen. &quot;Contraction and Backgrounding in Taiwan Mandarin.&quot; Concentric: Studies in Linguistics, vol. 32.1, 2006, pp. 69-88.