Ihatov gave you a good answer the first time you asked this question, so I’ll just go into further detail. Note that I’m just a learner of Chinese like you are. Normally, 知道 is pronounced zhi1dao0 (with 道’s inherent fourth tone reduced to the neutral tone, except that southerners may pronounce both syllables with full tone), however, when preceded by another character in certain expressions, such as 不知道, it is the 知 that loses its inherent first tone and is pronounced as a neutral tone. When this happens, as Ihatov mentions, the initial ZH often gets reduced to R: bu4ri0dao4. Native speakers may deny it, but you only have to spend 10 minutes listening to authentic material to hear this common expression and confirm the usual pronunciation. A similar example of this common phonological feature is 多少钱. Officially duo1shao3qian2 (or duo1shao0qian2), more typically duo1rao0qian2.