By rule, it's allowed. Those are suggestions and there is nothing preventing the PA announcer by rule to get the crowd fired up. But, the PA announcer isn't supposed to do this once the opposing team breaks the huddle and through the snap count. An official could flag the home team if the PA announcer or even loud artificial noise makers (music piped through or train horns) are used from the time the opponent breaks the huddle until the time the ball is snapped. Other noise makers like air horns are never flagged and they are fine. Basically, about any noise or any loud commenting by the PA announcer is allowed, but the exception is once the team breaks the huddle until the time the ball is snapped.