The chunk which hits San Francisco was reported to be traveling at about 32,000 miles per hour. That's very credible for an orbital collision. However, the firey blob we see impact with the city was clearly traveling at about 500 miles per hour. Something traveling at 32,000 will not be seen before it hits. There certainly won't be any time for someone to hear a rumble (speed of sound is only about 700 miles per hour - anything faster than that you won't hear before it arrives).
At 00:52:11 in the movie, an operator in a military surveillance control room is reading aloud live coordinates of a fast-moving object on his radar screen. The content of the screen is shown full screen for 2 seconds. However what the "radar" screen displays is a plain DOS listing of root directory on his computer hard disk. As the operator supposedly gives additional information about speed and destination of the moving object, what we see is that he types "DIR /W" within an After Dark DOS Shell session and yet again the root directory is displayed. We can even see clearly that Win95 is installed on the computer.