When it's used to modify an adjective, "molto" is an adverb and it doesn't change gender or number. Here, "molto" is the adverb modifying the adjective "rumorosa". If we made the noun in the sentence plural, we would still use "molto" (Quelle strade sono molto rumorose.)
When it's used to modify a noun, "molto" is an adjective and must agree with the noun in number and gender. So if we were saying, "There are many noisy streets," here "many" is modifying "streets" (not noisy) - Ce sono molte strade rumorose.
For further information, here's a thread from a Word Reference forum: http://forum.wordreference.com/showthread.php?t=313055