1. There must be a lot of noise from the traffic. - It's about something that you talk about for the first time, in this case 'the noise from the traffic', and you say that it exists. 'There' in this sentence doesn't mean the opposite of 'here'. It is 'there is', as in 'There is a market in town.' 2. It must be very noisy. - 'It' refers to a particular place, 'a busy road'. The overall meaning of the two sentences is the same. I hope it will help.

Both sentences are affirmative and hypothetical. Despite the first sentence describing the type of sonorisação (traffic noise) and the second sentence only generalize (noise), there was no change in understanding, making clear the information.